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Old 28th August 2014, 09:17 AM   #1
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Default How to increase gain of Quad 34 preamp.

Hello
Is there an easy way to increase the gain of the tape input of the Quad 34 preamp, say double.
Thanks
thyristor44
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Old 28th August 2014, 01:27 PM   #2
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There are a couple of versions of the Quad 34. Based on the second issue (all grey buttons) R16,R18 need only be replaced by wire links to increase sensitivity from 300mV to 100mV for full rated output. You can easily download the earlier model schematic and go from there if you want that. This change is specified on the schematic, though with a sensitivity of 10x that normally required for digital sources, noise will probably become quite noticeable.
http://www.hifiengine.com/manual_library/quad/34.s
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Old 28th August 2014, 04:12 PM   #3
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Thanks Ian I have already done that, the problem is gain with cd player and radio are fine but input from dac is much less and almost has to be on max vol for satisfactory level. Could be I need a better dac as some seem to have a gain control.
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Old 28th August 2014, 07:43 PM   #4
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by thyristor44 View Post
Hello
Is there an easy way to increase the gain of the tape input of the Quad 34 preamp, say double.
Thanks
thyristor44
Depends on your definition of easy

Its easy

Cut the print and insert a resistor ( RF on my diagram). 10k to 22k would be a good value. Select R to give as much gain as you want. Gain = (RF + R)/R so using say 15k for RF and 4k7 for R would give a gain of just over 4. Drop R to 1k and you have a gain of 16.

Edit... I've just spotted the input resistors too. If R137/138 are fitted then you have a large attenuator effect. Make sure they are not fitted. You could usefully link out the 39k's in series with the input imo.
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Old 28th August 2014, 08:33 PM   #5
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Nice idea Mooly, unfortunately its a very early 34 and they dont have op amps on tape input, all inputs are switched to one op amp and increasing gain here would increase all gains. I could add op amp to tape input as in your diagram but that is getting quite major for me.
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Old 28th August 2014, 09:23 PM   #6
sreten is offline sreten  United Kingdom
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Hi,

I'd look at increasing the output of the DAC,
can't be hard, couple of resistors probably.

rgds, sreten.
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Last edited by sreten; 28th August 2014 at 09:31 PM.
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Old 28th August 2014, 09:57 PM   #7
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Quote:
Originally Posted by sreten View Post
Hi,

I'd look at increasing the output of the DAC....
Agreed, this would be where to look because any freestanding DAC should be capable of 1V output, like any digital source, and should easily overdrive a 300mV tape input.

Unless it's attenuated in firmware or there are pots or fixed dividers preset for another duty, it must be malfunctioning. I shouldn't think there is a need for a better DAC if there is some output - just that it's output buffer level needs sorting out. If you keep raising preamp gain to compensate for feeble input, you only make matters worse.
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Old 29th August 2014, 11:40 AM   #8
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There are no dividers I can see. The outputs from the 9001 go through 33 ohm resistors and the outputs from the 1543 go to 1k resistors to earth also the Vref, 1k to earth.
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