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Tweaking the Quad 34s PSU

John_73

John_73

pfm Member
Hi folks.

After lightly modding, and recapping the line and phono stages of my Quad 34, (and with special thanks to all those who helped me recently adjusting the loading on the MC card to match my OC9 cartridge), my attention has turned to getting the best out of the PSU section.

The standard PSU isn't terrible, but I think with a replacement of both the generic 7918 regulator, and the bridge rectifier, lower psu noise levels might be of benefit, particularly for the sensitive MC phono stage section. I'm sure the line stages won't sound any different. I don't want to go mad with OTT changes, and end up rebuilding something that's no longer a Quad, but I think a few light tweaks might be worthwhile. I've already recapped the PSU using Panasonic FC and FM types, and C74 has been increased to 2200uF.

I intend to replace the 7918 regulator with one of these:

http://www.newclassd.com/index.php?page=70&hv=1

…and the bridge rectifier with four 11DQ10 diodes:

http://www.hificollective.co.uk/components/diodes.html

According to the instructions, the new regulator is a drop-in replacement for the 7918, but I was wondering whether any additional components (C67?) from the Q34s PSU section should be removed now too, components that might have been there just to ensure the stability of the stock 7918?

Thanks,

- John

image-1.jpg
 
Hi again quad2fan:

The QUAD 34’s PSU is already pretty good; to be honest there is very little to be gained by replacing the 7918-voltage regulator. QUAD knew what they were doing and they generally did it well. I am also sceptical about using Schottky & Ultra-Fast diodes and what benefit they would bring. In the overall scheme of things 50Hz is pretty slow and well with in the range of any ordinary diode.

On my unit I replaced C74, C58 and C84. These capacitors are the main “workers” in the circuit and would be the main candidates for improvement. C69 could also be replaced at the same time but its main operation is the output muting rather than any direct audio function so its job is not so critical.

There have been improvements in electrolytic capacitor technology since the QUAD was first made. Modern electrolytic capacitors are smaller these days so on my 34, I replaced C74 (1000uf) with a 50 volt 2200 uf low ESR cap and C58 & C84 (22uf) with 50 volt 47uf low ESR devices. After that, most other modifications would yield very little improvement but could risk upsetting the balance or over-work other parts within the PSU.

Happy Listening:
LPSpinner.
 
Russ Andrews used to have an upgrade scheme for this preamp. It recommended quite a few changes including inserting Schottky diodes. I performed all the changes on mine. The biggest difference came when I replaced all the caps that are in the signal path in series with the opamps. There are a lot of them.

I sold the unit ages ago. Have you swapped the opamps? It wonder how it would sound with some of the latest ones where distortion is negligible.
 
Thanks for the suggestions folks.

Just to re-cap (arf!) the line & phono stages are sorted; I already removed/shorted the caps DaDa recommend in their upgrade PDF, with the exception of the caps prior to the volume control. (Audiosmile/Baker Audio) Rob suggested best to leave those in place to prevent the possibility of any DC causing the volume pot to go noisy over time.

Phono stage & MC module (with Keith Snook mods) upgraded & singing away nicely.

TL071 op-amps retained; I like them, and feel they're fine in the line level sections IMHO. I have upgraded the CMOS switches however to MAX4066s. The phono stage op-amps replaced with LME49710HA; figured these might benefit from any PSU mods for lower noise I could get out of the Q34.

All PSU caps replaced with Panny FC/FM types. I've only increased the capacitance of C74 which is now 2200uF/50V. Following LPSpinners (ta for those!) comments I might increase C58 & C84 from 22uF to 47uF.

It's been my experience in the past that best bang for the buck is usually in upgrading the PSU. I might still give that upgrade regulator a go, as it's very easy to install, and restore to stock if I don't like it. I just needed to know if I should remove any caps before doing so (i.e. Compensation/stability caps necessary for the stock 7918 only).

Thanks,

John.
 
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