• WANTED: Happy members who like to discuss audio and other topics related to our interest. Desire to learn and share knowledge of science required as is 20 years of participation in forums (not all true). Come here to have fun, be ready to be teased and not take online life too seriously. We now measure and review equipment for free! Click here for details.

Electrostatic Headphone Amplifier Review & Comparison

kevin gilmore

Member
Joined
Feb 29, 2020
Messages
37
Likes
187
armir's srm-007 is a rev 0 unit (3 stax jacks on the front panel) AC filament. But that also means its a minimum of 25 years old.
If it has never been serviced, then highly likely that the power electrolytics are in serious trouble. I would think that would show up
as 60 and 120hz bumps in the spectrum. large bumps. The rating of this amplifier is 1350Vpp. If the 100k load (200k balanced) did nothing
additional bad to this amp, it would bring that rating down to 800Vpp. No one certainly tests dynamic amplifiers this hard.

marantz 8b is a very nice sounding push pull amplifier with transformer output. many transformer output amps don't like driving another transformer directly. meaning an amp like this with a srd7 is a bad idea.

Very few amplifier/srd7 combinations are going to be able to achieve .00x thd. Actually none that i can think of.
 

Robin L

Major Contributor
Joined
Sep 2, 2019
Messages
3,118
Likes
4,132
Location
1 mile east of Sleater Kinney Rd
marantz 8b is a very nice sounding push pull amplifier with transformer output. many transformer output amps don't like driving another transformer directly. meaning an amp like this with a srd7 is a bad idea.
'Splains a lot. However, I noticed a similar sound quality with speakers. "Very nice"? Probably too "nice".
 

kevin gilmore

Member
Joined
Feb 29, 2020
Messages
37
Likes
187
example why amplifier+transformer can't match direct drive amps
 

Attachments

  • marantz.jpg
    marantz.jpg
    421.9 KB · Views: 229

the_brunx

Active Member
Joined
Apr 7, 2020
Messages
183
Likes
400
I am impressed with the sheer power of the SRD-7 Pro & my Amp Combination. it can as cleanly as I'm able to hear, with bass eq'd up to the max, play so loud until the headphones themselves start rattling and still no sign of clipping unlike the SRM amps i had before. which would have already grated me to death with premature clipping distortion when EQ'd like this and driven this loud. and that was the most bothersome thing for me.
 
Last edited:

Degru

Active Member
Joined
Feb 19, 2019
Messages
230
Likes
225
Location
Beaverton, OR
I hooked my Stax headphones to a Marantz 8b with one of those. Very strange. Everything in the mids was holographic, deep bass was absent, as were the top two octaves. Best combo ever for Sarah Vaughan and Ella Fitzgerald records. Goof click, pop and rumble filter.
Sounds like something went wrong. What headphones, and which transformer? It should also be noted some amps have trouble driving a transformer/capacitive load. The impedance drops sharply in the bass down to 0.5 ohms at the extremes IIRC. The amp doesn't need a ton of gain, but it needs to have good current capabilities. The transformer is designed for around 30WPC amps, but 100WPC is what I'd recommend as a rule of thumb to make sure it's driven properly. I ran it out of a 50WPC Audiosource amp 100, which performs fine with normal speakers but makes the bass on stax quite a bit compressed. My current amp the Adcom GFA-545ii does rather well. I also got decent results from a lowly SMSL SA50, no compression but bass slightly boomy and overall obviously not as refined as the Adcom.

EDIT: Just looked it up, trying to drive it out of a 60s 35wpc tube amp might be the issue :p
 

Robin L

Major Contributor
Joined
Sep 2, 2019
Messages
3,118
Likes
4,132
Location
1 mile east of Sleater Kinney Rd
Sounds like something went wrong. What headphones, and which transformer? It should also be noted some amps have trouble driving a transformer/capacitive load. The impedance drops sharply in the bass down to 0.5 ohms at the extremes IIRC. The amp doesn't need a ton of gain, but it needs to have good current capabilities. The transformer is designed for around 30WPC amps, but 100WPC is what I'd recommend as a rule of thumb to make sure it's driven properly. I ran it out of a 50WPC Audiosource amp 100, which performs fine with normal speakers but makes the bass on stax quite a bit compressed. My current amp the Adcom GFA-545ii does rather well. I also got decent results from a lowly SMSL SA50, no compression but bass slightly boomy and overall obviously not as refined as the Adcom.

EDIT: Just looked it up, trying to drive it out of a 60s 35wpc tube amp might be the issue :p
Like I said, it basically sounded like amps of that era and general design always sound like to me, slightly underpowered, lacking in deep bass, nice in the midrange but blurry on top.
 
OP
amirm

amirm

Founder/Admin
Staff Member
CFO (Chief Fun Officer)
Joined
Feb 13, 2016
Messages
35,029
Likes
130,724
Location
Seattle Area
Personally I would like to see the same tests redone with a front-end as proposed by Kevin (can't be hard to build) and accuracy doesn't even have to be in the 0.000x % at all as that won't be reached without extreme open loop gains.
It is actually a pain in the neck to build the voltage divider. For a 10:1 attenuator, you need a 900K resistor to build a divider relative to 100 K inside the analyzer. That is an easy to find resistor in low voltage but in high voltage, it essentially does not exist in the market. I spent hours on usual suspect sites (digikey, mouser, etc.) and could not find it. Could use other divider ratios but then you have to do a mental math for the ratio.

The other issue is noise pick up with high impedance load as opposed to what I am using now.

A serious concern is overloading of the AP input. With the current 100K load, there is enough voltage drop to not cause a problem. But with higher impedance load, this is a concern.

All this said, I did purchase some 1 meg and 10 meg resistors to use in parallel. And some high voltage caps to deal with the AC impedance. Parts will come this weekend and I will see what I can do.

It is one of those projects where the value to membership is so tiny yet the hassle factor so high. If this was something we needed to do to test every headphone is one thing. But spending money and time just to test half a dozen products ever, it has terrible ROI.
 

Degru

Active Member
Joined
Feb 19, 2019
Messages
230
Likes
225
Location
Beaverton, OR
example why amplifier+transformer can't match direct drive amps
Seems like a pretty cherry-picked bad example. Something around 100wpc with good current capabilities and low noise floor would be more suitable. 35WPC is enough in terms of gain, but most 35WPC amps won't drive the load very well.
 

Robin L

Major Contributor
Joined
Sep 2, 2019
Messages
3,118
Likes
4,132
Location
1 mile east of Sleater Kinney Rd
Seems like a pretty cherry-picked bad example. Something around 100wpc with good current capabilities and low noise floor would be more suitable. 35WPC is enough in terms of gain, but most 35WPC amps won't drive the load very well.
Right. :rolleyes:

Look, I used what was at hand. Around 1995. I wanted to find out how the 8b sounded with the headphones and it didn't sound good. And that's all I really needed to know because I'm probably never going to listen to Stax headphones in the future for reasons I've already given.

Like Amirm sez, testing these highly specialized amps is a high irritation project with minimum actual usefulness for most folks.
 

solderdude

Master Contributor
Joined
Jul 21, 2018
Messages
9,857
Likes
21,366
Location
The Neverlands
It is actually a pain in the neck to build the voltage divider. For a 10:1 attenuator, you need a 900K resistor to build a divider relative to 100 K inside the analyzer. That is an easy to find resistor in low voltage but in high voltage, it essentially does not exist in the market. I spent hours on usual suspect sites (digikey, mouser, etc.) and could not find it. Could use other divider ratios but then you have to do a mental math for the ratio.

The other issue is noise pick up with high impedance load as opposed to what I am using now.

A serious concern is overloading of the AP input. With the current 100K load, there is enough voltage drop to not cause a problem. But with higher impedance load, this is a concern.

All this said, I did purchase some 1 meg and 10 meg resistors to use in parallel. And some high voltage caps to deal with the AC impedance. Parts will come this weekend and I will see what I can do.

It is one of those projects where the value to membership is so tiny yet the hassle factor so high. If this was something we needed to do to test every headphone is one thing. But spending money and time just to test half a dozen products ever, it has terrible ROI.

I don't know if 'simply' 900k in front of the AP input is such a good idea.
There is also 200pF of capacitance on the input. That would have to be switched off using 900k in front of it otherwise the 900k and 200pF will become a LPF at 884Hz.
Perhaps this isn't an easy fix and you may need a real frontend (high Z, low capacitance).

If it were my AP I would be very careful with it's input. High voltages can creep into funny places on a PCB.
 
OP
amirm

amirm

Founder/Admin
Staff Member
CFO (Chief Fun Officer)
Joined
Feb 13, 2016
Messages
35,029
Likes
130,724
Location
Seattle Area
I don't know if 'simply' 900k in front of the AP input is such a good idea.
There is also 200pF of capacitance on the input. That would have to be switched off using 900k in front of it otherwise the 900k and 200pF will become a LPF at 884Hz.
Perhaps this isn't an easy fix and you may need a real frontend (high Z, low capacitance).
Yeh, the capacitance is a crapshoot. For distortion though we just need to be good to 1 kHz so it may be fine.
 
OP
amirm

amirm

Founder/Admin
Staff Member
CFO (Chief Fun Officer)
Joined
Feb 13, 2016
Messages
35,029
Likes
130,724
Location
Seattle Area
Yeh, the capacitance is a crapshoot. For distortion though we just need to be good to 1 kHz so it may be fine.
Oh, we need bandwidth for harmonics as well so scratch that.
 

Degru

Active Member
Joined
Feb 19, 2019
Messages
230
Likes
225
Location
Beaverton, OR
Right. :rolleyes:

Look, I used what was at hand. Around 1995. I wanted to find out how the 8b sounded with the headphones and it didn't sound good. And that's all I really needed to know because I'm probably never going to listen to Stax headphones in the future for reasons I've already given.

Like Amirm sez, testing these highly specialized amps is a high irritation project with minimum actual usefulness for most folks.
I wasn't replying to you, I was replying to kevin making a generalization about transformer setups based on the measured distortion of one amp.
 

LTig

Major Contributor
Forum Donor
Joined
Feb 27, 2019
Messages
3,549
Likes
5,721
Location
Europe
It is actually a pain in the neck to build the voltage divider. For a 10:1 attenuator, you need a 900K resistor to build a divider relative to 100 K inside the analyzer. That is an easy to find resistor in low voltage but in high voltage, it essentially does not exist in the market. I spent hours on usual suspect sites (digikey, mouser, etc.) and could not find it. Could use other divider ratios but then you have to do a mental math for the ratio.
What about the brute force approach of using 9 x 100k resistors in serious? Each single resistor sees only 1/9th of the voltage.
And don't forget a small cap in parallel (or accordingly bigger ones in parallel to each single resistor) to compensate the input capacitance of the AP.
 
OP
amirm

amirm

Founder/Admin
Staff Member
CFO (Chief Fun Officer)
Joined
Feb 13, 2016
Messages
35,029
Likes
130,724
Location
Seattle Area
What about the brute force approach of using 9 x 100k resistors in serious?
You would increase chances of noise pick up with such a long string. Keeping such a long string insulated would be more challenging as well.
And don't forget a small cap in parallel (or accordingly bigger ones in parallel to each single resistor) to compensate the input capacitance of the AP.
Have to see if that is needed or not as the wiring and attenuator will have some capacitance to start. But yes, as I noted I have bought some high voltage caps as well.
 

brimble

Active Member
Forum Donor
Joined
Oct 18, 2020
Messages
115
Likes
136
Location
Australia
Not surprised. Pls, can you share what models you are referring to?.

I don't remember all the amps I've tried, but at least the following, all of which sounded identical to me (unlike the tube amps):
SRM-Xh
SRM-3
SRM-717 I think?
SRM-727 Mk II
 

solderdude

Master Contributor
Joined
Jul 21, 2018
Messages
9,857
Likes
21,366
Location
The Neverlands
Yeh, the capacitance is a crapshoot. For distortion though we just need to be good to 1 kHz so it may be fine.

You could consider paralleling a (very small) capacitance, maybe even a resistor in series with it, to compensate for the loss of higher frequencies.
The same thing is done with scope probes, here it usually is a small trimmer cap which is easily calibrated by looking at a 1kHz squarewave that is usually provided by the scope itself.

And of course you will need a 100-120pF 2kV or 4kV capacitor loading the amp (so before the 900k) in differential mode (not 2 caps to ground)
 
Last edited:

kevin gilmore

Member
Joined
Feb 29, 2020
Messages
37
Likes
187
The 1.9 meg resistor i linked to with (4 x 50pf/1kv ceramic in series) in parallel is a perfect match for a 20x attenuator compensated for the 100k and 200pf load. Rated at 2.5kv so plenty of headroom. And the resistor with caps soldered across it is only 1 inch long.

and you still need a 120pf capacitor equivalent as the load.

would be interesting to see the results of the benchmark ahb2 driving a srd7 measured before and after the srd7
 
Last edited:

Maki

Active Member
Joined
Oct 24, 2018
Messages
249
Likes
443
everyone talking about thd+n from the hp
I don't remember the THD results too well but I think there was no difference at my listening volume, although my environment at the time wasn't ideal.
 
Top Bottom