Restoring a Threshold FET 10e - a few questions - Page 2 - diyAudio
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Old 27th July 2011, 04:28 AM   #11
a.wayne is offline a.wayne  United States
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Quote:
Originally Posted by GaryB View Post
ZM,
The gain module for the phono stage uses C3, C4, and C8. The line stage does not use them. I did replace C8 on the phono modules. The photo below is taken from a.wayne's pictures of his FET 9e but it uses the same gain modules as the 10pe. C8 is the electrolytic in the bottom left corner, while C3 and C4 are the mica caps along the bottom edge of the board below the transistors with heat sinks.

Regarding the question of whether or not some drift of the DC level of the phono modules is "normal", I'm thinking that it must be since Threshold includes a 10uf output cap even though the modules are servo'd.

One other thing to note is that the schematic differs from the board in one respect. The schematic in the 1st post doesn't show any adjustments. But there is a square blue ten turn pot on the board just above the servo op amp. At first I thought that this pot might allow control of the bias current. Tracing out the circuit, it turns out this is in series with R10 - the output resistor from the servo op amp. My guess is that this resistor is set so that the op amp is operating roughly in the middle of its voltage range.

As I stare at the circuit, I realize that I don't fully understand how the servo works. I'm used to seeing servos fed into the "-" input. But this is being fed into the current sources for the input stage. Nominally, the bases of transistors Q3 and Q4 are fixed by the turn on voltage of D3 / D4. But changing the op amp output will cause more or less current to flow through the diodes and slightly change the voltage of D3 / D4. Which in turn will increase the input current and the current of the next stage. I don't see how the loop closes. Can someone explain how this helps zero the output?

Thanks,
---Gary
It looks exactly like a Fet9 phono stage ...
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Old 27th July 2011, 06:35 AM   #12
GaryB is offline GaryB  United States
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Quote:
Originally Posted by a.wayne View Post
It looks exactly like a Fet9 phono stage ...
That's because it's a picture of your FET9 phono stage that I took from another thread. I mentioned it in the text that you quoted. Here are some snap shots of the FET10pe. It uses 2 gain modules per channel and of course is a stand alone phono stage, unlike the FET9, which uses 1 gain module per channel and integrated phono and line stage in one chassis.
---Gary
phono1.jpg phono_mid.jpgphono3.jpg
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Old 28th July 2011, 01:28 AM   #13
GaryB is offline GaryB  United States
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Default one more pretty picture

Thought this one turned out reasonably well.
---Gary
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Old 28th July 2011, 02:01 AM   #14
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You might think about upgrading the external power supply.
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Old 28th July 2011, 03:45 AM   #15
GaryB is offline GaryB  United States
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Originally Posted by john curl View Post
You might think about upgrading the external power supply.
John,
It's definitely on my list of next steps now that I've replaced the old electrolytic and tantalum caps. I already rebuilt the power supply with slightly larger caps (15k uf --> 22k uf), diodes upgraded to Fairchild soft recovery, snubber added across transformer, and some film bypass caps added. Before and after pictures below. I've also got a larger transformer LC supply that I'll patch in for comparison. If that sounds better, which I expect, then I'll build another set of dedicated supplies.

---Gary
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File Type: jpg 10e_ps.jpg (500.9 KB, 379 views)
File Type: jpg Recapped_10ePS.jpg (286.8 KB, 314 views)
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Old 28th July 2011, 05:45 AM   #16
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Sounds like a good start. We have found that this particular preamp really benefits from a strong power supply, meaning big caps, big transformer and high speed diodes. Sold one to Gordon Holt once, just from an A-B listening test.
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Old 28th July 2011, 07:14 PM   #17
a.wayne is offline a.wayne  United States
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Originally Posted by GaryB View Post
That's because it's a picture of your FET9 phono stage that I took from another thread. I mentioned it in the text that you quoted. Here are some snap shots of the FET10pe. It uses 2 gain modules per channel and of course is a stand alone phono stage, unlike the FET9, which uses 1 gain module per channel and integrated phono and line stage in one chassis.
---Gary
Attachment 232883 Attachment 232884Attachment 232885
Well one works betta than 2.... .......err not ..Would the 2 gain stages allow balanced line operation and In beefing up the power supply, would double the transformer rating be sufficient with the bigger caps ..?

Last edited by a.wayne; 28th July 2011 at 07:17 PM.
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Old 28th July 2011, 10:08 PM   #18
GaryB is offline GaryB  United States
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Quote:
Originally Posted by a.wayne View Post
Would the 2 gain stages allow balanced line operation and In beefing up the power supply, would double the transformer rating be sufficient with the bigger caps ..?
See the schematic in post #2. The 10pe runs two gain modules in parallel for each channel, presumably for lower noise. You can't use them for balanced operation.

Regarding beefing up the power supply, I've built a supply with a 160va transformer that I'm going to try out. Others have reported benefits from going even bigger - say 250va or 300va.

---Gary
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Old 24th September 2011, 11:17 PM   #19
GaryB is offline GaryB  United States
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Others have reported benefits from going even bigger - say 250va or 300va.

---Gary
OK - I've finished with my power supply experiments and as usual, John Curl is right. This preamp really benefits from a hefty power supply.

First I compared the restored PS (see post #15) to a 160VA LC power supply that I had lying around. There was no comparison - the LC supply was much better. Both supplies had 22,000uf capacitance and roughly the same output voltage. But the transformer in the LC was ~ 2x the VA rating of the stock supply.

With that promising results, I set out to build a really beefy power supply. I got some 18v/18v 300VA toroid transformers from Antek and tried various permutations. First I tried an RC-LC supply, which didn't sound particularly good.
The values I used were:
R = 5ohms
C1 = 4700uf
L = 0.32H / 10 ohms
C2 = 22,000uf

I should have gotten a higher voltage toroid to compensate for the IR drops in the R and L. This supply only had 1v margin over the 18v output of the preamp regulators - not enough. Shorting out the R to make this a CLC supply raised the voltage to 22v and sounded much better. It was now competitive with the LC supply. While 22v (4v margin) should be plenty of margin for the preamp regulators, it's still lower than the stock supply which puts out close to +-24v. I decided to short out the inductors and see how that sounds. It would make the ripple worse but the regulators would have more margin. Vout rose to ~ +-23.3v and the sound got much better. The sound really opened up and became more dynamic with a bigger soundstage.

The final supply is the 300VA toroid 18v/18v to separate Fairchild soft recovery bridges (one for plus and one for minus) to 4700uf + 22000uf. I included snubbers on the transformer secondaries to damp out any diode reverse recovery noise. And I bypassed the electrolytics with 10uf, 1uf, and 0.2uf Russian military caps.

I can bring this to next week's Burning Amp festival if anyone is interested in hearing the difference this kind of supply makes compared to the stock supply.

---Gary
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Old 30th September 2011, 08:15 PM   #20
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Hi Firends,

I have not yet read the Gary B. feedback on this topic, but we have collaborate together : you can read a full description of the upgrade that I have done on my own Fet-10HE preamp on my website, here:



I will comment later when I have read this topic, but for sure, upgrading the Power Supply brings a huge improvement.

Other upgrades, and particularly replacing the original NOBLE potentiometers with attenuators (I have used GOLDPOINT attenuators as described in the article) brings the same huge improvement...
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