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Old 27th February 2012, 12:47 AM   #1
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Default ASL Preamp Help

I want to do a little tube rolling in a ASL AQ 2006DT preamp. I have no idea where to start though. There are 4 12ax7s and 2 12au7s along with a 6BM8 and a 5ar4. I'm pretty sure the rear pair of 12ax7s are for the phono stage as theory are covered by tube shields but what do the others influence? I'll attach a couple of schematics.
Thanks in advance
Dave
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Old 27th February 2012, 03:13 AM   #2
kevinkr is offline kevinkr  United States
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The rest are part of the line stage which IMHO seems needlessly complex.
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Old 27th February 2012, 10:12 AM   #3
DF96 is offline DF96  England
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That is a strange circuit! Appears to have a rather basic phono preamp, followed by a complicated line stage. The line stage has a cathode coupled amp (sometimes wrongly called 'grounded grid') followed by a cascaded cathode follower, although the diagram is rather difficult to read.

I can't tell which way round the line stage valves are, but either way would be wrong! If the single 12AU7 (per channel) is used for the cathode coupled stage then the 12AX7 is used for the follower and you have weak current drive capability. The other way round you have plenty of drive, but far too much gain.

As there is no overall negative feedback, and the phono stage probably has barely enough loop gain for bass boost, the performance of the circuit will depend on how closely the valves you have match those used by the designer.
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Old 27th February 2012, 10:42 AM   #4
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Quote:
Originally Posted by DF96 View Post
That is a strange circuit! Appears to have a rather basic phono preamp, followed by a complicated line stage. The line stage has a cathode coupled amp (sometimes wrongly called 'grounded grid') followed by a cascaded cathode follower, although the diagram is rather difficult to read.

I can't tell which way round the line stage valves are, but either way would be wrong! If the single 12AU7 (per channel) is used for the cathode coupled stage then the 12AX7 is used for the follower and you have weak current drive capability. The other way round you have plenty of drive, but far too much gain.

As there is no overall negative feedback, and the phono stage probably has barely enough loop gain for bass boost, the performance of the circuit will depend on how closely the valves you have match those used by the designer.
None of the above sounds promising. Doesn't SOUND too bad though. Not using the phono stage. Running my DIY phonoclone3 into the dac input.
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Old 27th February 2012, 12:24 PM   #5
DF96 is offline DF96  England
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Your DAC may have better drive capabilities than the preamp. If used with cables which are not too long the main effects of the preamp will be to add some unwanted gain (which has then to be attenuated away in the volume pot) plus a little noise and distortion.
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Old 27th February 2012, 12:45 PM   #6
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The dac is just another input. No special circuitry as far as I know. I just use it as another input.
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