2. amps-preamps
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Gain Matching - need advice

I recently added an Audio Research Ref 6 to my system (it was a dealer demo unit so price was right).  The preamp is paired with Pass Labs XA 60.8 amps (26db gain and 100K ohm input impedance) and Avantgarde speakers (sensitivity of 108 db at nominal 8 ohms).  The problem I am having, that I did not have with my previous Joule Electra LA150, is volume control.  The AR preamp has a 103 position volume control.  At position #7 the volume is almost ear splitting.  Sure I can turn it down but it’s very difficult to dial in a suitable volume.  I am inclined to blame it on the new AR preamp (output impedance of 600 ohms and 12db of gain).  I am stuck into using balanced interconnects which I won’t go into.  My first conclusion was that I have a gain matching problem since the old preamp did not behave this way.  Looking at the difference between the output impedance of the preamp and input impedance of the preamp ... why would there be a gain matching problem (at least as I understand the subject ... which I may not.  Anything that can be done about this, or a new preamp the only solution?
chilli42
georgehifi's avatar
georgehifi
10,312 posts
 


You need to throw some gain away, obvious is to dump the preamp/s. And use a 10kohm passive pre so anything can drive it. (save for some silly high output impedance sources) (usually tube).

Forget inline attenuators, too much of a gamble, you never know if you have an impedance match or not, (unless you know how to measure them) because from what I was told they don’t state what series/shunt resistances they have used inside them

Cheers George
speakermaster
738 posts
 
You do have a gain problem because of the having to use balanced interconnects it gives you 12 decibels more gain than what you think 6 db from the pre amp and amp each that would give you 32 plus 18 which is 50 db of gain way too much on an avantgarde if you could hook it up single ended you would have 38 db of gain ant that would be ok on the avangardes i would think but maybe still a little too much but nothing like the 50 db you have now.
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signaturesound
83 posts
 
Gain spec of Joule preamps in product lit are not accurate. I have actually measured many different Joule preamps.... in default shunt to ground volume control configuration, a Joule LA-150 has little bit more than unity (i.e a few tenth more than 0db IIRC) gain at max volume. Switching the preamp volume control jumpers to voltage divider got you around 6dB.
CheersRich
chilli42 OP
111 posts
 
Thanks all for the advice.  As a first step, I am going to try the Rothwell Attenuators.  If they work that’s great, if not then I am not out that much $.  I looked into having AR modify the preamp but they say this can only be done on older models.  If the attenuators don’t work then it’s going to need something more drastic ... preamp or speaker change.  Unbalance interconnects are a non starter for me as I have a very long lead between amp and preamp ... and for reasons I won’t go into I can’t change this.  I also discovered that my Lumin X1 DAC/streamer has an analogue volume setting of normal or low.  The low completely takes care of the matching problem but there is a big change in bass volume ... which I can’t explain.  Again, thanks to everyone who responded!


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