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Old 11th May 2008, 04:36 AM   #11
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The biggest improvement I experienced when modding my 34, was to short circuit all the bypass caps in the circuit, replacing only C77 and C78 in the output.

As I recall I did manage to get this nice little pre to run dead flat from 3Hz to 300KHz
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Old 11th May 2008, 08:11 PM   #12
johnm is offline johnm  United Kingdom
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Thanks for the tips guys

Hi Jan - Yep I've already bypassed the caps, and - like you - this really cleaned things up ALOT. Bypassing the balance control as well also helped a great deal. I never have had the need for balance controls so I don't miss it. Those tone controls can be handy on occasion though so I've left those in-circuit.

Vivavee, if I implement this RC network which side of it would goto the plus, and which to the minus in each case? I currently use 100uF Elna Silmic there. Maybe I should try bypassing that with a small value cap?
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Old 11th May 2008, 08:39 PM   #13
VivaVee is offline VivaVee  New Zealand
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If you are referring to C77 and C78, then the resistor can go either side. It does not matter electrically, but there may be a better side in terms of physical placement on the PCB. I am at work at the moment and don't have access to the PCB. Just remember to preserve the polarity of the electrolytic caps.
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Old 11th May 2008, 08:59 PM   #14
VivaVee is offline VivaVee  New Zealand
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I have drawn my suggestions for the power supply modification, as I'm sure my words are inadequate. See the attached file, I hope.
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File Type: png quad 33 mods.png (78.6 KB, 238 views)
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Old 11th May 2008, 09:43 PM   #15
johnm is offline johnm  United Kingdom
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Thanks so much for that diagram - that's great! I'll do that tomorrow

Would you be able to do a quick diagram of the C77/C78 mod as well to make sure I have it correct?

That 1000uF cap in the PSU has been replaced by a 2200uF Panasonic FC. I take it the PSU mod you show in the diagram us compatable with this?

Thanks again!

- John

P.S. As to N1 it doesn't appear to be any diagram, but it's in both Quad 34s I own. It's between neutral and live on the input, and is a combined Rifa 0.047uF capacitor/ 100ohm resistor in series, in potting compound. I think it's to lesson on/off pops. Removing it didn't seem to make a difference.
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Old 11th May 2008, 10:30 PM   #16
VivaVee is offline VivaVee  New Zealand
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See the pic for C77 mods.

N1 is a snubber - you won't notice any difference until you start turning lights on/off on the same circuit as your preamplifier. The snubber will diminish the volume of the crack you hear through the speakers. So just leave it there.
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Old 11th May 2008, 11:38 PM   #17
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Thanks ever so much for that Vivavee! Now it's clear

I'll do those tomorrow and report back on how they sound.
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Old 21st May 2008, 04:31 PM   #18
johnm is offline johnm  United Kingdom
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Vivavee - thanks for your mods. Implemented the PSU mod today and it does appear to have cleaned up the sound a tad. Changing to Shottky rectifiers helped as well.

I haven't attempted the C77 / C78 mods yet as I'm happy with the sound at the moment (using 100uF Elna Silmics here), but once the itch to mod comes on again I'll certainly try it

Cheers,

- John
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Old 21st May 2008, 05:51 PM   #19
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Interesting....
Almost inspires me to get my already modded 34 up from the cellar and try some further moddings
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Old 23rd May 2008, 08:22 PM   #20
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I really do think it's a fantastic pre-amp now, and - believe me - I know why people used to mock its sound.

I sounds like a passive pre now..... with balls

I'd say that hand-matching resistors between channels, and upgrading the caps and switching bank ICs probably had the most effect, then the op-amps, then the PSU.
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