Fosi Audio GR70 Quad Tube Headphone Amp / Preamp

[WaxMaster]

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 am sure you have spent a considerable amount of time with your new Fosi GR-70 by now. Were you as stunned as I was by it's sound quality as both a headphone amplifier and preamplifier? I spent a large sum of money trying to find the right combo of tubes to use with the GR-70, so let me save you some time an money regarding possible tube upgrades. As much as I love the sound of my Raytheon JRP-6AK5 black plate tubes and RTC/Mullard M8100's, nothing compares to the Voshkod 6ZH1P-EV Rocket tubes I purchased from Yen Audio on eBay. With the Voshkod tubes in the input stage and a NOS pair of 1944 RCA JAN-CRC-6AC7 output tubes, I am swimming in a immersive sea of holographic music. I have everything I could possibly hope for with this combination of tubes. The input tubes seem to have the biggest impact on the sound quality of the GR-70. Do not skimp here and just buy the Voshkod Rocket tubes and thank me later. I just did an A/B demo between the JRP-6AK5's, M8100's & Voshkod 6ZH1P-EV's playing music from my Qobuz " Speaker, Cable & System Burn-in Tracks " public playlist through an Eversolo DMP-A6 Master Edition Gen.1 (w/Eddy Pardo Special Edition Linear PSU). This is connected to a Geshelli Labs JNOG 3 (AK4499EX) dac via a Cardas Clear USB cable. The JNOG-3 has Sparko's SS2590 Pro op-amps on the XLR outputs and a Virtual Hi-Fi Hypersonic op-amp on the rca outputs. One pair of AQ water rca's connect the JNOG-3 to the Fosi GR-70 & another run of AQ Water's connect the Fosi to my my Kali IN-5 studio monitors. The headphones that I am using are Verum 1 MKII's, Verum 2's and Sivga SV-021 dynamic headphones. The sound I am hearing from this desktop setup is sublime. The Fosi just makes beautiful music when the right combination of vacuum tubes are installed. After just 10 hours of playing time on Voshkod Rocket tubes, I have absolutely no interest in listening to any of my other 6AK5 tubes. The final upgrade will be a Fiio PL-70 linear psu when it is released in early 2026. This will replace to stock ac/dc wall wart power adapter that ships with the Fosi. I will also purchase a 2nd GR-70 as a backup "spare" in the event that anything ever happens to the one I use now. I have no desire to upgrade any further. The Fosi GR-70 checks all of the boxes for me, and I love the fact that it was super affordable. I wish my other gear offered the same price to performance ratio as the Fosi does.

 

[solderdude]

cool story with lots of impressive name dropping all condensed in a WOT.

 

[Alexium]

Has anyone been able to measure the output impedance? I wonder about its feasibility for IEMs.
This seems to not be an OTL amp, see the two small(ish) transformers:

Spoiler

 

[dvp]

• Is there any chance it will drive DT 880 600 Ohm 96 db?

 

[Vitarta]

I am sure you have spent a considerable amount of time with your new Fosi GR-70 by now. Were you as stunned as I was by it's sound quality as both a headphone amplifier and preamplifier? I spent a large sum of money trying to find the right combo of tubes to use with the GR-70, so let me save you some time an money regarding possible tube upgrades. As much as I love the sound of my Raytheon JRP-6AK5 black plate tubes and RTC/Mullard M8100's, nothing compares to the Voshkod 6ZH1P-EV Rocket tubes I purchased from Yen Audio on eBay. With the Voshkod tubes in the input stage and a NOS pair of 1944 RCA JAN-CRC-6AC7 output tubes, I am swimming in a immersive sea of holographic music. I have everything I could possibly hope for with this combination of tubes. The input tubes seem to have the biggest impact on the sound quality of the GR-70. Do not skimp here and just buy the Voshkod Rocket tubes and thank me later. I just did an A/B demo between the JRP-6AK5's, M8100's & Voshkod 6ZH1P-EV's playing music from my Qobuz " Speaker, Cable & System Burn-in Tracks " public playlist through an Eversolo DMP-A6 Master Edition Gen.1 (w/Eddy Pardo Special Edition Linear PSU). This is connected to a Geshelli Labs JNOG 3 (AK4499EX) dac via a Cardas Clear USB cable. The JNOG-3 has Sparko's SS2590 Pro op-amps on the XLR outputs and a Virtual Hi-Fi Hypersonic op-amp on the rca outputs. One pair of AQ water rca's connect the JNOG-3 to the Fosi GR-70 & another run of AQ Water's connect the Fosi to my my Kali IN-5 studio monitors. The headphones that I am using are Verum 1 MKII's, Verum 2's and Sivga SV-021 dynamic headphones. The sound I am hearing from this desktop setup is sublime. The Fosi just makes beautiful music when the right combination of vacuum tubes are installed. After just 10 hours of playing time on Voshkod Rocket tubes, I have absolutely no interest in listening to any of my other 6AK5 tubes. The final upgrade will be a Fiio PL-70 linear psu when it is released in early 2026. This will replace to stock ac/dc wall wart power adapter that ships with the Fosi. I will also purchase a 2nd GR-70 as a backup "spare" in the event that anything ever happens to the one I use now. I have no desire to upgrade any further. The Fosi GR-70 checks all of the boxes for me, and I love the fact that it was super affordable. I wish my other gear offered the same price to performance ratio as the Fosi does.

Hi. Thanks for interesting info. I want try but have one question: what exactly writed on 1944 RCA JAN-CRC-6AC7. I found a couple but with different VT. As example. VT150, VT112 and etc. And see without VT writed on lamp. So many variants. Please help me

 

[Connor1a]

Hi

The GR70 takes 2 types of tubes: two of the type 6AC7 tubes and two of the type 5654W tubes.

With respect to the 6AC7, that’s its commercial designation. You may also find them listed as a VT112, which is their military designation, or you may also see them listed by the description in your post “RCA JAN-CRC-6AC7” where: 1944 was the manufacturing year, RCA was the manufacturer, JAN = Joint Army / Navy, CRC = Coordinating Radio Committee (US Navy’s specifications - means that specific tube was made for the US Navy), and then 6AC7 = the tube type number ("6" indicates it has a 6.3-volt heater, "A" designates it as a sharp-cutoff pentode, and "C7" is its specific tube type). The 6AC7 is listed as having a number of direct replacements (6J4, 1852, and various Russian and Chinese versions like the 6*4 and 6J4P), but in my search, I found few of these available.

The 5654W is a bit easier. The 5654W is the commercial version of the tube. The VT112 is the military version. From what I read, even through the tubes are direct swaps, the 5654W is thought to be the superior tube. The 6AK5 / 6AK5W are also military versions of the tube also thought to be lower performers vs the 5654W. Other direct swaps are the EF95, 403B, 6J1, and 6J1P. I didn’t find a lot on those.

The only reference to a VT150 I saw referred a 6SA7 vacuum tube which is used in type of bidirectional radio receivers (ie likely not something used in amplifiers / preamplifiers). NOT what you want to use here. Perhaps you transposed the digits when searching for the 6AC7…?

End of the day, I am NOT an expert in this field. Take what I say with a grain of salt. I’m sharing things I learned when I went through this myself. There are a number of sites on the internet that are dedicated to vacuum tube technology in modern sound. TheTubeStore.com and TubeDepot.com are two of the better known. They’re slightly (to substantially) more than eBay where you can also find a huge selection of tubes, but then, TheTubeStore.com and TubeDepot.com have been around for some time, do their own testing / matching etc… You get what you pay for…

I hope this helps a bit. Good luck!

 

[WaxMaster]

Hi. Thanks for interesting info. I want try but have one question: what exactly writed on 1944 RCA JAN-CRC-6AC7. I found a couple but with different VT. As example. VT150, VT112 and etc. And see without VT writed on lamp. So many variants. Please help me

Connor1a gave an excellent breakdown of the 6AC7 tubes and identifiers. You may also see the 1852 designation on 6AC7 tubes from the early 1940's. Later versions dropped the 1852 identifier and went with 6AC7 & VT-112 identifiers. I just acquired a large lot of 175 NOS 1943 USN JAN-CRC-6AC7 tubes in red/black RCA factory boxes. These also have the VT-112 logos on them. These are the best sounding 6AC7 tubes I have heard yet in the GR-70.

 

[Vitarta]

BIG THANKS all of you for so many detailed info. You are Real good people ready for help.
It is not expencive so I will try

 

[Connor1a]

You may also see the 1852 designation on 6AC7 tubes from the early 1940's

I had a professor at grad school who studied Russian patronymics as a hobby. Not familiar with patronymics? Neither were we few wee tots… A patronymic is a name derived from the father or ancestor such as Johnson meaning “the son of John”. Anyway, anytime we wanted out of a topic, we’d wind him up about Russian patronymics and he’d be off like a spinning top for at least an hour on the topic… He knew his detail and wasn’t afraid to use it…. And evidently the Russian naming system is quite complex. Anyway…

I find vacuum tube part numbers to be a similar experience. The 6AC7 son of VT-112 son of 1852 son of… I went out and searched my notes again and found several more that I missed for the 6AC7. Now whether they’re on the market is a completely different discussion, but evidently they handed out designators for vacuum tubes like new surnames on Ellis Island in 1907.

My advice is to ring one of the two tube sites above and ask them for options in your price range and use case. They’re both reputable and will give you some steering you won’t (likely) get on eBay. Not all of the tubes are true 100% replacements. For example, the 1852 I listed above is actually an industrial tube. It works in consumer grade electronics, but there may be trade-offs. For example, it may not be a perfect physical fit. The guys at one of the tube stores can guide you through that. You’ll pay a couple of extra dollars if you buy there (looked like they were both 2$ or 3$ more than eBay prices on the 6AC7), but you can make returns at both (though there is a 15% to 20% “restocking fee” to cover their cost of retesting / rematching tubes and so forth - I think that’s steep, but its better than mostly no returns on eBay). So net, both will cost you more than eBay, but they (arguably) have a bigger selection, they claim to do their own testing / matching and they have a return policy. I also believe they’re both US stores if that makes a difference (import/export fees etc.) and have US inventories. That leaves Amazon. Amazon is the most expensive I found, but they have the fastest shipping and free 30 day returns. I received a cracked envelope (glass container) on a tube I bought from Amazon. They took it back at full price without so much as a cough. There may be others and YMMV.

Last thing, these things can be pretty tight when they go in / come out. Someone here on the board suggested just a bit of dielectric grease on the tips of the tube’s pins might help them go in easier. I looked the suggestion up on an electronics board. The members there were hesitant. The gel could just as easily foul up the connection with the tube socket if too much is used. Another suggestion was to use rubbing alcohol or contact cleaner and a soft toothbrush to gently clean each of the pins before use (dirt accumulates during manufacturing, storage, testing which could hinder insertion - some of these tubes are from the 1940s). I personally chose option 2.

End of the day, I believe I bought a pair of NOS JAN (USN) VT-112 RCA tubes from eBay + a pair of Riverstone Audio NOS JAN 5654W Platinum tubes from Amazon. I’ve since given the preamp to my son-in-law who now refuses to admit he has it (for fear I’d take it back - he just loves it)…

A bit more to help you on your chase…

 

[ChefBrunch]

I have the GR70, sounds great IMO, I've used it with Sennheiser HD660 and Koss pro 4aa , also used it as a preamp for the brzhifi class a 20wpc amp with the mod done on it....its a nice pairing with Micca speakers ...but thats all much I tried it with.

 

[Vitarta]

У меня в аспирантуре был профессор, который в качестве хобби изучал русские отчества. Не знакомы с отчествами? Мы тоже не были малышами… Отчество — это имя, образованное от имени отца или предка, например, Джонсон означает «сын Джона». В общем, всякий раз, когда мы хотели уйти от темы, мы заводили его разговорами о русских отчествах, и он, как волчок, рассуждал на эту тему не меньше часа… Он знал свои тонкости и не боялся их использовать… И, очевидно, русская система имён довольно сложная. В общем…

Я нахожу, что с обозначениями электронных ламп происходит то же самое. 6AC7 — сын VT-112, сын 1852 года… Я снова просмотрел свои записи и нашёл ещё несколько, которые пропустил для 6AC7. Вопрос о том, есть ли они на рынке, — это уже совсем другой вопрос, но, очевидно, в 1907 году электронные лампы получили обозначения, как новые фамилии на острове Эллис.

Мой совет — позвонить на один из двух сайтов с лампами, указанных выше, и попросить их о вариантах в вашем ценовом диапазоне и варианте использования. Оба они авторитетны и дадут вам некоторую консультацию, которую вы (вряд ли) найдете на eBay. Не все лампы являются стопроцентной заменой. Например, упомянутая мной выше лампа 1852 года на самом деле является промышленной. Она работает в потребительской электронике, но могут быть компромиссы. Например, она может не идеально подходить по размеру. Ребята в одном из магазинов ламп могут вам помочь. Вы заплатите пару долларов дополнительно, если купите там (кажется, цены на 6AC7 там были на 2–3 доллара выше, чем на eBay), но вы можете сделать возврат на обоих сайтах (хотя есть «сбор за возврат» в размере 15–20 %, чтобы покрыть расходы на повторное тестирование / повторную настройку ламп и так далее — я думаю, что это дороговато, но это лучше, чем почти полное отсутствие возвратов на eBay). Итак, оба варианта обойдутся вам дороже, чем на eBay, но у них (возможно) выбор больше, они заявляют, что сами проводят тестирование и сопоставление, и у них есть политика возврата. Кроме того, я полагаю, что оба магазина находятся в США, если это имеет значение (импортно-экспортные сборы и т.д.), и у них есть американские запасы. Остаётся Amazon. Amazon — самый дорогой из найденных мной вариантов, но у них самая быстрая доставка и бесплатный возврат в течение 30 дней. Мне прислали треснувший конверт (стеклянный контейнер) на тюбике, купленном на Amazon. Они приняли его обратно по полной цене, даже не помявшись. Возможно, есть и другие варианты, и YMMV.

И последнее: эти штуки могут довольно туго входить и выходить. Кто-то из участников форума предложил нанести немного диэлектрической смазки на кончики контактов трубки, чтобы облегчить их установку. Я посмотрел это предложение на форуме по электронике. Участники форума засомневались. Гель может легко испортить соединение с гнездом трубки, если использовать его слишком много. Ещё один совет: аккуратно очистить каждый контакт перед использованием спиртом или очистителем контактов и мягкой зубной щёткой (грязь скапливается во время производства, хранения и испытаний, что может затруднить установку — некоторые из этих трубок ещё 1940-х годов). Лично я выбрал вариант 2.

В итоге, я, кажется, купил пару ламп NOS JAN (USN) VT-112 RCA на eBay и пару ламп Riverstone Audio NOS JAN 5654W Platinum на Amazon. С тех пор я отдал предусилитель своему зятю, который теперь отказывается признать, что он у него (боится, что я его заберу обратно — он просто в восторге)…

Еще немного, чтобы помочь вам в вашем преследовании…

Big big thanks. I already bought it on eBay. I hope everything will be okay. I'll write when I try it.

 

[Vitarta]

Sorry for maybe stupid question)))
But can I add Sparkos Labs ss3602.
Or what we can change for better sound exept lamps

 

[Connor1a]

I don’t believe the GR70 has any upgradable opamps. Your tone shaping has to take place with the tubes (which manny here don’t believe actually affect the sound quality). YMMV.

 

[Vitarta]

I don’t believe the GR70 has any upgradable opamps. Your tone shaping has to take place with the tubes (which manny here don’t believe actually affect the sound quality). YMMV.

So where is truje about lampa? I think one way - just try ten decide

 

[WaxMaster]

Connor1a is correct in stating that there are no swappable op-amps inside the Fosi GR-70. You can only alter the sound quality by changing out the vacuum tubes. I have over 150 total hours on my GR-70 since purchasing it and I have had a slight change of heart with regards to the Voshkod 5654 Rocket tubes. Yes they still sound amazing, however my pair started to get noisy at around 50 hours of use. Lately, I have been relying on my RTC/Mullard 5654RT's and Raytheon JRP-6AK5 tubes as they perform flawlessly in the GR-70 and are very quiet sounding input tubes.

 

[Vitarta]

Hi everyone! Arrived. Is everything ok and I can use? I have 6z1p-ev.
But was writed before in post about 6zh1p-ev.
It the same lamp and audio quality. I want ask before installing. Please help me

 

[Vitarta]

I tried gr70 as preamp to jds O2 old amp and it is another higher level. With gr70 and new lamps was good but with O2 sound just amazing. Recommend!