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Old 14th December 2012, 01:56 PM   #1
xtaljo is offline xtaljo  United Kingdom
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Default Connecting Two Amps to Common Input

Apologies if this has been covered previously. I want to use an Arcam 75 amp. as my sub-woofer amp. taking its input from the left output of a (passive) preamp. At the same time my main (Mullard 3+3 valve) left channel amplifier will take its input form the same source (ie paralleling the inputs to the two amps).

I have tried a quick arrangement and it works ok but,of course, the volume on the left channel main amp. has to be set higher than that on the right channel.
This isn't a problem but is their likely to be a serious impedance mismatch with this sort of connection? Is there a better way of driving two amps. from the same source?

Help very much appreciated.
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Old 14th December 2012, 02:06 PM   #2
DF96 is offline DF96  England
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Add a buffer. I mean a genuine buffer (high input impedance, low output impedance, gain=1, low noise, low distortion), not the FX boxes often sold as buffers. Basically some form of follower: low distortion active device with 100% feedback.
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Old 14th December 2012, 02:57 PM   #3
xtaljo is offline xtaljo  United Kingdom
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Many thanks for your advice.

I'm not at all familiar with buffer amps. Could you recommend a commercially available one that would be suitable? I can follow circuit diagrams for constructional purposes so if its a matter of building one could you point me in the direction of a suitable circuit?

Thanks again - I'm learning a lot.
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Old 14th December 2012, 03:07 PM   #4
DF96 is offline DF96  England
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The textbook unity gain non-inverting opamp configuration will do the job. Or you could use an ECC88 as a cathode follower, if you prefer valves.
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Old 14th December 2012, 03:32 PM   #5
xtaljo is offline xtaljo  United Kingdom
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Would I use two of these? Both connected to the left channel output of my passive pre-amp and one providing the input to the main left channel amp. and the other providing input to the sub-woofer amp?
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Old 14th December 2012, 03:50 PM   #6
jrenkin is offline jrenkin  United States
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Couldn't you use a minidsp to do this? Then it could do your XOs too.
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Old 14th December 2012, 03:54 PM   #7
DF96 is offline DF96  England
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You need just one buffer for the left channel. Both amps connect to its output.

However, for good stereo imaging you need to add a buffer to the right channel too. A good buffer does very little to the signal, apart from buffering, but it can't do nothing at all so you want the same 'almost nothing' done to both channels.
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Old 14th December 2012, 09:23 PM   #8
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And you really want to derive your subwoofer drive from both channels. A resistive summer after the buffers is fine.

All good fortune,
Chris
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Old 14th December 2012, 11:22 PM   #9
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I was waiting for someone to say it! Why would you want subs derived from only one side? Where/how do you plan to cross over, line level or speaker level, active or passive? Are you going to run the main speakers full-range? You could use a resistor sum on the bass output of an active crossover to drive the sub.

I'm surprised you had to adjust the volume on the side with the extra tap; I would have expected all input impdances to be so much higher than the drive impedance tha you could just drive the extra amp in parallel with almost no drop in voltage, just splitting the current. What's the input impedance of the Arcam?

There's other options, depending on what you want to do. You could go balanced and use splitter transformers, but you sound like a minimalist with your passive pre. And then if you want both channels to send to the bass you should buff to sum anyway.
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Old 15th December 2012, 07:37 AM   #10
xtaljo is offline xtaljo  United Kingdom
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Many thanks for all the suggestions.

Cyclecamper wrote:
Quote:
Originally Posted by cyclecamper View Post
I was waiting for someone to say it! Why would you want subs derived from only one side? Where/how do you plan to cross over, line level or speaker level, active or passive? Are you going to run the main speakers full-range? You could use a resistor sum on the bass output of an active crossover to drive the sub.

I'm surprised you had to adjust the volume on the side with the extra tap; I would have expected all input impdances to be so much higher than the drive impedance tha you could just drive the extra amp in parallel with almost no drop in voltage, just splitting the current. What's the input impedance of the Arcam?

There's other options, depending on what you want to do. You could go balanced and use splitter transformers, but you sound like a minimalist with your passive pre. And then if you want both channels to send to the bass you should buff to sum anyway.
Yes I am a bit of a minimalist and the use of only one channel stems from the availability of just one sub-woofer amp. and the fact that I read that many recording companies record low frequencies monaurally to save space (reflecting the fact that we can't locate very low frequencies anyhow). I know this is disputed by some but ||I like to keep things simple if possible..

Cross-overs (hi-pass) are in series with the main speaker inputs and a low- pass filter is in series with the Arcam and the sub-woofer. I am interested in the resistive summer idea as suggested by Chris but am not sure I quite understand it. How do you calculate the resistor values and is their a source that explains its use in a hifi contexts?

I do need to increase the volume on the Mullard amp side that is in parallel with the Arcam sub-woofer amp. in order to get a reasonable channel balance. I looked in the Arcam handbook and it gives the line input impedance as 12K (sensitivity 540mV). The input impedance of the Mullard 3+3 is not given in the literature but input goes across a 500K pot if that helps at all. The sensitivity of the Mullard is quoted as 100mV.

I'd like to keep things simple and avoid additional transformers really.

It looks like I need to get/build a couple of buffer amps. to match the impedances more closely but if the resistive summer idea is simpler I'd like to follow it up. But I'm not sure if its an alternative or an addition. Please excuse my lack of knowledge and worn-out brain!

Thanks again for the very helpful responses.

Last edited by xtaljo; 15th December 2012 at 07:44 AM.
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