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EAR 834P mod - help identifying components
EAR 834P mod - help identifying components
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Old 11th July 2018, 06:32 PM   #21
spaceistheplace is online now spaceistheplace
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EAR 834P mod - help identifying components
Quote:
Originally Posted by soulmerchant View Post
The valves/tubes are in a closed metal enclosure, so no need to do extra shielding.
enclosure is vented I believe. I’ve found shields on V1 especially in Phono to be always advisable if going for best measured results

Quote:

There is honestly only so much to get out of this topology. It can be pushed pretty far, but at some point you might want to go for something different.
Seems you have a suggestion in mind... what is it?
The riaa is tricky... easily changed by a few picofarad.

[/quote]

Yes but I was able to attain .1db with selected triodes without deviation from the values indicated above.

Quote:
Jfets from the same batch tend to exhibit similar characteristics. Mosfets however are a different story... From my experience there will be far greater potential variation between input 12ax7 triodes than the jfets.
Correct but you are assuming ideal circuit and properly selected JFETs.

Given the valve quality in these devices it’s not a stretch to assume non-ideal JFETs or that they’re implemented in a way that could be improved upon.

Chinese clones aren’t known for their build quality, attention to detail or similarity to the production schematics they’re scamming.

Anyhow, it’s the piece of the puzzle with the least known info so I find it worthwhile to investigate even for the sake of curiosity.

Quote:

What is the psu transformer like? The original ear834p was quite modest. Be happy you have no hum.
Again, V1 shield helps in this regard.

Likely a 50VA toroid. A larger toroid would increase mechanical hum likelihood, not reduce it.

Hum is best mitigated through attention to grounding practices or installing a snubber to reduce ringing.

Quote:
Honestly, if it sounds good, don‘t mess it up too much. If it is really not good enough, think about a different topology...

Good advice but this is diyaudio after all, he’s engaged in the learning process.

There’s value in his interest.
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Old 12th July 2018, 05:40 AM   #22
soulmerchant is offline soulmerchant  Switzerland
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Hi space

The original ear834p enclosure was not vented. But I am certainly not against using shields. I have some nice soviet sockets with shields here, but I would need a very pressing reason to de-solder the originals and solder in new ones with shields.

The problem with the few picos making a difference means that layout is really important. You can easily add a few picos on a circuit board alone by doing nothing and you might never know it. If you are getting even within 0.5dB of RIAA (measured using a good inverse RIAA network and frequency analyzer) then you are doing pretty good imho. Excellent job. I would gently note that passive RIAA is also easy enough to do and moves you into the nano farad range.

Also, as your input ecc83/12ax7 ages, your once close RIAA tolerance is sadly bound to drift. Swap the ecc83/12ax7 out for a new one and you will never know how good the close RIAA tolerance is. Maybe it is "good enough for rock and roll" though. I suspect most people just get used to a certain "sound" and are happy. That's fine too.

I don't know if others experience this, but measuring ecc83's on my digitally calibrated AVO MKII is honestly not such a treat. It is not easy to find ones that closely match channel to channel on transconductance. Such small ma/V values require lots of concentration. Amplification factors vary also channel to channel. I mention this since I found "tube rolling" really necessary with this circuit, but also found the need to closely measure them before hand, which was quite tedious. The ecc83/12ax7 is indeed very linear and an excellent voltage amplifier. However for the first stage in a one step RIAA correction it should be a well selected example (again just my opinion).

Sorry that you misunderstood my question on the power transformer. It has nothing to do with hum. I was only commenting that no hum is nice to have for what looks like a very faithful clone of this phono amplifier.

The question of the power transformer was in order to see what other PSU possibilities he might have. If it is a modest low VA torroid feeding a voltage doubler (as in the original ear), then major changes to the PSU will be a challenge. That is a pity since I experienced a significant improvement with this circuit was after improving the PSU.

IMHO there are better approaches to RIAA - but all with their own merits and costs. This circuit is definitely fine to start with and of course this is a diyboard. I am glad we agree on that.

Ian
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Last edited by soulmerchant; 12th July 2018 at 05:58 AM.
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Old 12th July 2018, 01:13 PM   #23
Herr Grau is offline Herr Grau  Germany
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Thanks to work, I had no time at all yesterday to get any work done on the amplifier. When I do, I will take a look at the toroid... I have witnessed many an unmarked Transformer in Chinese Amps though...
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