Fosi Audio GR70 Quad Tube Headphone Amp / Preamp

[Connor1a]

Wondering if anyone has tried the new Fosi Audio GR70 Tube HPA (preamp). I bought one from Amazon to give it a try. Comes in tomorrow. Pure, gratuitous tube madness.

https://www.amazon.com/Fosi-Audio-GR70-Tube-Amplification-Preamplifier/dp/B0FB3P3RZT

My “test” will be to run it as follows:

WiiM Pro (RCA Out) —> (RCA In) Fosi GR70 Preamp (RCA Out) —> (RCA In) Topping L30 II HPA (1/4” out) —> Sennheiser HD 660S2

I’m not sure that the L30 II is necessary, but I read the GR70 can struggle driving some higher impedance cans. If not, the L30 may go back to Amazon. Will have to see how it goes. Thought seriously about picking up the Topping D50 III as a DAC for the L30 II. That may be a next payday kind of thing. Were that to happen, I think the chain would be:

WiiM Pro (S/PDIF Out) —> (S/PDIF In) Topping D50 III (RCA Out) —> (RCA In) Fosi GR70 Preamp (RCA Out) —> (RCA In) Topping L30 II HPA (1/4” out) —> Sennheiser HD 660S2

Would love to hear your thoughts…

Peace…

 

[DVDdoug]

but I read the GR70 can struggle driving some higher impedance cans.

I don't know anything about this particular amp but generally tubes are "naturally" higher impedance and normally higher voltage than solid state so I'd be more concerned with low impedance headphones. The Amazon link says it's good down to 16-Ohms so low impedance may not be an issue either.

But even high-impedance headphones don't need what we'd call "high voltage". (5V from USB power might be too low.)

When you're driving 4 or 8-Ohm speakers almost all tube amps have an output transformer (which adds to the cost and makes it more difficult to make a good amp).


...I'm not a fan of this outdated technology. When I was a kid in the 1960's we had tube radios and tubes in our TV, and there was some transistorized stuff around. In the 1970's I had a car from the 50's with an AM tube radio. ...It was a cool-fun car but it had a 6V electrical system so I couldn't install a stereo.

 

[whataboutmusic]

Wondering if anyone has tried the new Fosi Audio GR70 Tube HPA (preamp). I bought one from Amazon to give it a try. Comes in tomorrow. Pure, gratuitous tube madness.

https://www.amazon.com/Fosi-Audio-GR70-Tube-Amplification-Preamplifier/dp/B0FB3P3RZT

My “test” will be to run it as follows:

WiiM Pro (RCA Out) —> (RCA In) Fosi GR70 Preamp (RCA Out) —> (RCA In) Topping L30 II HPA (1/4” out) —> Sennheiser HD 660S2

I’m not sure that the L30 II is necessary, but I read the GR70 can struggle driving some higher impedance cans. If not, the L30 may go back to Amazon. Will have to see how it goes. Thought seriously about picking up the Topping D50 III as a DAC for the L30 II. That may be a next payday kind of thing. Were that to happen, I think the chain would be:

WiiM Pro (S/PDIF Out) —> (S/PDIF In) Topping D50 III (RCA Out) —> (RCA In) Fosi GR70 Preamp (RCA Out) —> (RCA In) Topping L30 II HPA (1/4” out) —> Sennheiser HD 660S2

Would love to hear your thoughts…

Peace…

I did picked it up.

Tube sound on very low cost...decent materials and build quality too.

I haven't tried it as a preamp yet but it does a good job with the RME as a dac only...I have the 650's, 660s and the ndh 30's and it compliments all of them with the sound signature it provides. Though EQ from RME dominates things when it is applied...but retaining the whole tube sound in the end.

Cheers

 

[Lefteris]

I’m extremely disappointed with the support I received from Fosi Audio. When their product failed, they took an unreasonably long time to respond, and ultimately refused to stand behind what they sold. Despite clear evidence of the issue, they dismissed their responsibility and offered a token $30 as compensation — a gesture that felt more like an attempt to silence the complaint than to resolve it.


What’s most concerning is their disregard for basic consumer rights. Rather than addressing the problem or offering a meaningful solution, they chose to deflect and delay. This experience has left me with no confidence in their customer service or their commitment to quality.


I would strongly caution others to consider this before purchasing. A company that doesn’t support its own products is not one I can recommend.

 

[IT3ngineer]

Here's my story with Fosi GR70 and my experience with first ever tube amp/pre-amp.

Ordered mine through Amazon, had it for a week, when I decided to look into the weird startup sequence the amp had.

It would flicker on and off the tubes and it would make a click noise everytime I would start it up (unless the tubes were already warmed up). I watched some videos online and saw that's not normal and it should be slowly just warming up.

Reached out to Fosi Audio Support and they were quick to respond, after I've provided video evidence where they confirmed it's a PSU issue and decided to send me one. A week passed and then another issue apperead where one channel started experiencing pops, and I'd confirm it would be the tubes as swapping them would move the issue to another channel, so I decided to return the unit within the 30 days period.

I've told them I was disappointing in the product and they offered a discount if I purchase another one and so I did.
The 2nd unit arrived, which is even worse than the first one.

One whole channel isn't working at all and its tied to the tubes. I'm now returning that 2nd unit and staying away from this amp until they fix their QC or something else.
As I believe that the amp might be good, but the tubes they've used are definitely bad.

I know they said they're vintage and may show signs or wear and stuff, but they're straight up on the brink or dying.

I wish someone else didn't had my rotten luck and enjoys this amp as I did actually like the sound when it worked properly.

 

[Multicore]

Would love to hear your thoughts…

WiiM into tube stage - very liberal. Good for you.

 

[whataboutmusic]

I haven't experienced any problems so far I am around 40 hours of the tubes to be honest. I also really like the sound it is refreshing and fun..

But that is not good...what does ''vintage'' means? so if we need a replacement they do may not have stock? In their website they only have the little ones..what about the bigger black ones?

 

[Saponetto]

I'd like to see the circuitry of this device...
If it's actual full tube (pre + power stage) as it appears, it should be an OTL amplifier with known limits with low impedance headphones.

 

[Connor1a]

Here's my story with Fosi GR70 and my experience with first ever tube amp/pre-amp.

Ordered mine through Amazon, had it for a week, when I decided to look into the weird startup sequence the amp had.

It would flicker on and off the tubes and it would make a click noise everytime I would start it up (unless the tubes were already warmed up). I watched some videos online and saw that's not normal and it should be slowly just warming up.

Reached out to Fosi Audio Support and they were quick to respond, after I've provided video evidence where they confirmed it's a PSU issue and decided to send me one. A week passed and then another issue apperead where one channel started experiencing pops, and I'd confirm it would be the tubes as swapping them would move the issue to another channel, so I decided to return the unit within the 30 days period.

I've told them I was disappointing in the product and they offered a discount if I purchase another one and so I did.
The 2nd unit arrived, which is even worse than the first one.

One whole channel isn't working at all and its tied to the tubes. I'm now returning that 2nd unit and staying away from this amp until they fix their QC or something else.
As I believe that the amp might be good, but the tubes they've used are definitely bad.

I know they said they're vintage and may show signs or wear and stuff, but they're straight up on the brink or dying.

I wish someone else didn't had my rotten luck and enjoys this amp as I did actually like the sound when it worked properly.

Here’s where I am today.

I currently have 3 units here at my home. Two boxed for return to Amazon. One I’m using w/ my headphones as described above. To get to a working unit, I had to jump through several hoops to Frankenstein something together.

1) I purchased a pair of matched, NOS 5654W tubes Riverstone Audio (~25$) on Amazon. These are the 2 small vacuum tubes on the front of the amp.

2) I then took the best looking set of 6AC7 tubes from the 3 GR70s I have. Happened to be a set of tubes marked USN (I’m assuming US Navy). These are the tubes in the metal casing.

3) I then used those two sets of tubes in a pristine GR70 chassis, powered it up and tested. I’ve had zero issue with it thus far. Well, I have some hiss coming through on some audio, but as its inconsistent, I’m assuming that its the recording(s) vs the preamp or my A50III/D50III or HD660S2s.

4) I have a forth GR70 winging its way from China from Fosi’s factory. I told their tech support that I wouldn’t accept anything less than a fully tested replacement (originally, they only wanted to send me a new chassis without tubes).

My guess is that, as you’ve already pointed out, they’re dealing with “vintage” tubes which aren’t being matched across the channels / across the tubes and the tubes are old / not in ideal shape so the failure rate is higher. Once again, Fosi is exposing their customers to their unique QA process. Perhaps they should be burning these tubes in at the factory for a period of time and then individually testing. I honestly don’t know, but I think I had to go to ridiculous lengths to get to a working unit. I really should have walked away by now already… I’m a glutton for punishment evidently…

 

[Connor1a]

very liberal

Liberal? I’ll never say, but tell me this, if things weren’t meant to connect, why do I have so many boxes of cables??? <grin>

 

[Connor1a]

In their website they only have the little ones..what about the bigger black ones?

I spent some time looking for the 6AC7 tubes (the larger, black ones) myself. There are stocks of them coming out of Russia, China and the Slovak Republic. You can find them here in the US as well. The trick is finding a reputable supplier who you can count on to supply you with higher quality “new old stock” (NOS) / matched sets (sets of tubed with close to matching electrical operating parms). The pair I’m currently using are marked USN (as in US Navy). Search for 6AC7 NOS Matched Pair and you’ll see several sites pop up carrying the tubes. I’d reach out to them before buying as the prices range from 4$/tube to 30$/tube (though most of it seems to be shipping cost). Mismatched vacuumed tubes can affect your sound quality. I have no idea how much Fosi does to ensure their sets of tubes are matched. Could be a lot, but given my experience, I’ll likely lean toward them not doing a lot…

Here’s a site for you:

Search: 33 results found for "6ac7"

Best vintage audio vacuum tubes, (valves) for tube preamps, tube amplifiers, phono stages, stereos, guitar amplifiers, microphones, recording studio equipment, radios, ham radios, jukebox; GE, RCA, Mullard, Telefunken, Western Electric, Sylvania, Raytheon, Westinghouse, Tungsol, Siemens...

tubeworldexpress.com

Check eBay as well (though that’s where shipping gets quite expensive)… YMMV

Edit - Just off of a discussion with the guy who owns Tube World. His opinion, buy a set of matched RCA NOS vacuum tubes. Beyond that, there’s limited / no sonic difference between the 6AC7 tubes… That was his opinion.

 

[Multicore]

Liberal? I’ll never say, but tell me this, if things weren’t meant to connect, why do I have so many boxes of cables??? <grin>

The cables don't care but digital into tube is not ... how should I say? ... orthodox.

If you're starting with digital then I say emulate tubes. The software modelers are very good and not expensive.

And you can the adjust the sound to taste for each individual track you play. Do you really want the same effects applied to everything indescriminately?

 

[whataboutmusic]

I spent some time looking for the 6AC7 tubes (the larger, black ones) myself. There are stocks of them coming out of Russia, China and the Slovak Republic. You can find them here in the US as well. The trick is finding a reputable supplier who you can count on to supply you with higher quality “new old stock” (NOS) / matched sets (sets of tubed with close to matching electrical operating parms). The pair I’m currently using are marked USN (as in US Navy). Search for 6AC7 NOS Matched Pair and you’ll see several sites pop up carrying the tubes. I’d reach out to them before buying as the prices range from 4$/tube to 30$/tube (though most of it seems to be shipping cost). Mismatched vacuumed tubes can affect your sound quality. I have no idea how much Fosi does to ensure their sets of tubes are matched. Could be a lot, but given my experience, I’ll likely lean toward them not doing a lot…

Here’s a site for you:

Search: 33 results found for "6ac7"

Best vintage audio vacuum tubes, (valves) for tube preamps, tube amplifiers, phono stages, stereos, guitar amplifiers, microphones, recording studio equipment, radios, ham radios, jukebox; GE, RCA, Mullard, Telefunken, Western Electric, Sylvania, Raytheon, Westinghouse, Tungsol, Siemens...

tubeworldexpress.com

Check eBay as well (though that’s where shipping gets quite expensive)… YMMV

Edit - Just off of a discussion with the guy who owns Tube World. His opinion, buy a set of matched RCA NOS vacuum tubes. Beyond that, there’s limited / no sonic difference between the 6AC7 tubes… That was his opinion.

Thanks for the detailed response and for sharing the information

I appreciate it

 

[Connor1a]

The cables don't care but digital into tube is not ... how should I say? ... orthodox.

If you're starting with digital then I say emulate tubes. The software modelers are very good and not expensive.

And you can the adjust the sound to taste for each individual track you play. Do you really want the same effects applied to everything indescriminately?

Sorry for the delayed response.

To be fully transparent, I originally picked up the GR70 as more of a toy—I just wanted to hear what it would do in my headphone chain. My real goal was the D50III —> A50III pair (which has absolutely delivered).

That said, once I found a set of tubes that worked and had some time to tinker, the GR70 turned out to be pretty cool. I’ve especially enjoyed the discrete bass/treble knobs. To your point, they’re simple but genuinely useful—letting me shape the sound per track or mood. Not as powerful as a full 10-band PEQ, fair enough, but fun all the same.

So, is the GR70 worth the $190 price tag? Based on my experience, I’d say no—that’s because of the time and effort it took to get a working setup. But once it was up and working, it’s met (and maybe even exceeded) my expectations.

YMMV.

 

[Multicore]

I’ve especially enjoyed the discrete bass/treble knobs.

Yes! Underrated feature in hifi these days. Graphics and PEQs are all rather too complicated even with a very good user interface. You actually need to train yourself to understand what such EQs do. Bass and treble knobs are way easier.

 

[Jeromeof]

So I have been enjoying the GR70 with some of my headphones for the past few days, so today I decided measure the effect of the High-Z and Low-Z toggle with some headphones.

I think its possible these measurements might be interesting, especially the effect with different headphones with different impedances.

So, basically the headphones stayed on the measurement rig and were not moved during these measurements:

Biggest difference was with my HD600:

I think many people would like this extra bass boost on the HD600.

This roughly matches the impedance curve of the HD600:

I think this is interesting as we don't often see the measured effects of a tube-amp on a headphone with a specific impedance curve.

Next, I tried a Sivga Peng (as it has an impedance of 340 ohms) - but the difference in this case was minimal:

but that closed back headphone has some FR issues which sadly where not enhanced with the GR70.

And finally I tried with a low impedance headphone, the Meze 105 Silva as I have been enjoying that headphone the past few weeks:

The High-Z removed allot of SPL and then when I level matched the output there was still some subtle differences in the treble.

Overall, I have a lot of headphone amplifiers that do not change the FR ( other than with EQ) so it can be nice to have one that does, for example here are a few aligned measurements other amplifiers (Aune N7, Burson Playmate 3 and even an Apple usb-c dongle) with the HD600 being measured not moved:

Aligning them you can see its only the GR70 that changes the FR.

 

[whataboutmusic]

So I have been enjoying the GR70 with some of my headphones for the past few days, so today I decided measure the effect of the High-Z and Low-Z toggle with some headphones.

I think its possible these measurements might be interesting, especially the effect with different headphones with different impedances.

So, basically the headphones stayed on the measurement rig and were not moved during these measurements:

Biggest difference was with my HD600:
View attachment 476240
I think many people would like this extra bass boost on the HD600.

This roughly matches the impedance curve of the HD600:
View attachment 476241

I think this is interesting as we don't often see the measured effects of a tube-amp on a headphone with a specific impedance curve.

Next, I tried a Sivga Peng (as it has an impedance of 340 ohms) - but the difference in this case was minimal:
View attachment 476239
but that closed back headphone has some FR issues which sadly where not enhanced with the GR70.

And finally I tried with a low impedance headphone, the Meze 105 Silva as I have been enjoying that headphone the past few weeks:
View attachment 476243
The High-Z removed allot of SPL and then when I level matched the output there was still some subtle differences in the treble.

Overall, I have a lot of headphone amplifiers that do not change the FR ( other than with EQ) so it can be nice to have one that does, for example here are a few aligned measurements other amplifiers (Aune N7, Burson Playmate 3 and even an Apple usb-c dongle) with the HD600 being measured not moved:

View attachment 476247

Aligning them you can see its only the GR70 that changes the FR.

Thanks for confirming what I was hearing with my hd650's on the high Z setting.

With the lower impedance neumann's (I think 120Ω) the result with high Z is a way more quiet and "anemic" sound...I don't know why though.

The amp does add to the sound signature by a lot....I really like very pleasant and easy to listen to sound.

I wonder if using the incorrect setting e.g. using the 650's with the low Z setting...would that be bad for the amp or the headphones?

 

[Connor1a]

Your Neumanns are indeed at 120Ω. The Sennheisers are at 300Ω.

The short answer, to stay “safe” (no volume / bass degradation or distortion), is to divide your headphones’ impedance by 8 (aka “the 1/8th rule”) and keep your amp’s impedance below whatever number that is. In my case, I have the Sennheiser 660S2s. Their impedance is 300Ω. Divide that by 8 = 37.5Ω. I should be able to safely use any amp with an output impedance below 37.5Ω (my Topping A50III is well within that range—as is your ADI 2).

Here’s a (much) more comprehensive answer:

Headphone & Amp Impedance

INTRO: The output Impedance of headphone sources is one of the most common reasons the same headphones can sound different depending on wha...

nwavguy.blogspot.com

Hope this helps.

PS — How do you find Tidal? Have you compared it vs Amazon and Qobuz? I’m also on Tidal. I’m iffy on the interface… I like that Qobuz has the option to buy and download FLAC / WAV versions of albums online. Have you tried the other HIRES music sources?

 

[whataboutmusic]

Your Neumanns are indeed at 120Ω. The Sennheisers are at 300Ω.

The short answer, to stay “safe” (no volume / bass degradation or distortion), is to divide your headphones’ impedance by 8 (aka “the 1/8th rule”) and keep your amp’s impedance below whatever number that is. In my case, I have the Sennheiser 660S2s. Their impedance is 300Ω. Divide that by 8 = 37.5Ω. I should be able to safely use any amp with an output impedance below 37.5Ω (my Topping A50III is well within that range—as is your ADI 2).

Here’s a (much) more comprehensive answer:

Headphone & Amp Impedance

INTRO: The output Impedance of headphone sources is one of the most common reasons the same headphones can sound different depending on wha...

nwavguy.blogspot.com

Hope this helps.

PS — How do you find Tidal? Have you compared it vs Amazon and Qobuz? I’m also on Tidal. I’m iffy on the interface… I like that Qobuz has the option to buy and download FLAC / WAV versions of albums online. Have you tried the other HIRES music sources?

Yes RME can handle almost every headphone both ndh 30 and 650 sound good with it.

I would expect that 650 would sound better than the ndh30 with the tube amp....but it is the opposite...ndh30 sounds amazing with the gr70.

As for Tidal I like it...where I live there is no Qobuz option but people say it has better catalogue on specific genres and maybe better sound quality. I have tried spotify and I don't prefer it compared to Tidal....I wish Tidal had its algorithm for new music. I have tried some hi-res albums(purchased them online) and I personally don't hear any difference compared to Tidal's hi-res files. I do hear a difference between normal and hi-res files of the same album on tidal but it doesn't mean that the high-res version is always better.

 

[solderdude]

So I have been enjoying the GR70 with some of my headphones for the past few days, so today I decided measure the effect of the High-Z and Low-Z toggle with some headphones.

I think its possible these measurements might be interesting, especially the effect with different headphones with different impedances.

So, basically the headphones stayed on the measurement rig and were not moved during these measurements:

Biggest difference was with my HD600:
View attachment 476240
I think many people would like this extra bass boost on the HD600.

This roughly matches the impedance curve of the HD600:
View attachment 476241

I think this is interesting as we don't often see the measured effects of a tube-amp on a headphone with a specific impedance curve.

Next, I tried a Sivga Peng (as it has an impedance of 340 ohms) - but the difference in this case was minimal:
View attachment 476239
but that closed back headphone has some FR issues which sadly where not enhanced with the GR70.

And finally I tried with a low impedance headphone, the Meze 105 Silva as I have been enjoying that headphone the past few weeks:
View attachment 476243
The High-Z removed allot of SPL and then when I level matched the output there was still some subtle differences in the treble.

Overall, I have a lot of headphone amplifiers that do not change the FR ( other than with EQ) so it can be nice to have one that does, for example here are a few aligned measurements other amplifiers (Aune N7, Burson Playmate 3 and even an Apple usb-c dongle) with the HD600 being measured not moved:

View attachment 476247

Aligning them you can see its only the GR70 that changes the FR.

In high-output Z mode the output impedance must be around 700Ω (like some old speaker amps with headphone out can be) in order to get a 3dB boost in the lows in the HD600 (when rig mounted).

Below HD600 on 0.2Ω and 120Ω

I think many people would like this extra bass boost on the HD600

Yep, that's why the HD650 was made (HD600).