Pioneer SA-610 - bypass the Pre-amp stage?

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dakku

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Joined 2013
#1
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#4
In the OFF position of the tone controls the signal will be not affected by them, so you can use any inputs (except of course phono) feeding the power amp stage directly, only the volume and balance controls in the way. S3 (loudness), and S4 (AC/DC coupling) should be in their OFF positions.
 
#5
Hi Dragonweed.
Thank you for the reply and explanation. Is there any way to bypass the volume and balance so I can control this from my audiolab pre? Basically, I want to use this amp as a power amp! Is it possible?
 
#6
Not having the actual schematic it is s bit difficult.... Anyway: you can use let's say the tape monitor input as the input for the power amp directly. Locate the common point of S4 (AC/DC coupling selector switch), remove the original coupling cap from the upper leg (coming from the volume pot), then wire the monitor input directly to the common point, which is the actual input of the power amp. For safety's sake better use a good quality cap (if you are not 100% sure that your preamp has 0V DC at its output) for coupling, likely a few uF/100V polyprop will do, if the input impedance of the poweramp is not too low.
 
#7
Like this....
 

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#8
While you can use just the power amplifier that way (as long as tone control is turned off), that still doesn't really make the unit a power amp - it combines the gain of pre and power into one stage. Voltage gain still is 42 dB, vs. a more typical 26 dB for a pure power amp. This could become rather noisy, and even with a resistor divider (or when leaving the volume control in), I would imagine that amp noise itself wouldn't be terribly low.

Reducing voltage gain may be possible considering that things have to remain stable even with the highs turned all the way down, but it is not something I would recommend without having the necessary means to verify performance and stability.
 
#9
Well, finally got a schematic for your amp, and the situation is not too simple, it uses a kind of split feedback, where the tone control circuit is one part of the fb loop. Unless you have good skills in amp mods, this will not be easy (as I thought first based on the block diagram of the amp). :whazzat:. Reducing the power stage gain here is more than swapping two resistors.....
 
#10
Thank you sgrossklass and dragonweed for taking the time in explaining the situation. I am not as familiar with amp circuits so I really appreciate your help.

Am I right in thinking that I can use my preamp to feed this amp if I :
- fed the signal via tape in
- left the volume to 50% (12o clock) position
- turned off the switch for loudness gain
- turned off the switch for Subsonic thingy
- turned off the switch for Tone Control

would that work?
 
#11
Yes, it should work like that without any mod, but be aware as sgrossklass said, the gain of the poweramp is rather high so if you are driving it from a separate preamp, you will easily overdrive it. With 40 dB of gain you need about 2-300mV eff. at the input to get full power, and your preamp will pump out much more than that even at moderate volume setting. As you see on the original gain diagram the power stage is driven without any gain stage prior to the volume pot.
 
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