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In Reply to: My suggestion... posted by Triode_Kingdom on March 12, 2007 at 20:56:10:
I've also been thinking about a frontend pot. Obviously adding one wouldn't reduce the amplifier noise passed to the speakers, but it sure would reduce, to the extent it's turned down, the preamp linestage noise being amplified by the amp.Also, I recall vaguely that using the 4- and 8-Ohm output taps for common and positive, respectively, gets one 2 Ohms; is that correct? If so, the Voltage gain thru those taps would be even less than thru the 4-Ohm taps. I guess I'll measure that.
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Tin-eared audiofool and obsessed landscape fotografer.
http://community.webshots.com/user/jeffreybehr
Follow Ups:
Using the wrong taps will reflect a mismatch back to the 845. Sound quality will almost certainly be compromised. It's also impossible to know how the transformer will perform (frequency response, ringing, etc.) when the lead intended by the manufacturer to be used a common is left open.I'll add that the SRPP modification is probably the only simple means to reduce gain by an appropriate amount without degrading S/N. A stepdown transformer at the input will reduce linestage noise, but high sensitivity speakers will still reproduce internal noise of the amplifier's front end. On the other hand, bypassing the first stage altogether will reduce amplifier gain too much (23 dB). I think you would need a high level source to compensate for such a large reduction.
I'll start with the 2nd, since that will reduce preamp and poweramp-1st-gain-stage noise, while removing the 1st cap will (presumably) reduce only preamp noise.Again, thx for your help. I'll let you know what happens.
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Tin-eared audiofool and obsessed landscape fotografer.
http://community.webshots.com/user/jeffreybehr
It's the easiest way of reducing gain and way better than a pot IMO. It will change the sound a little and you may or may not like it. You should also know that removing the bypass cap will also degrade PSRR by almost 50% and increase the OP impedance by a similar amount but neither should be a factor in your circuit.Actually the first stage is not an SRPP at all since the drive is taken from the plate of the bottom tube. It's more like a symmetrical CCS grounded cathode stage but the 1K plate R provides a very poor load. I'm not a fan of this configuration because it forces the use of the same value plate R as the cathode R and this compromise leads to a quiescent current and a plate R that are both too low for good linearity. It also (normally) forces you to bypass the cathode R for good gain and low OP impedance. It would be simple to improve by adding a few components.
For the solution to the specific problem you seek TK's suggestion is the best. For other benefits with simple mods consider also 1) taking the drive from the cathode of the top triode and 2) increasing the value of all cathode and plate Rs to between 1K5 and 2K2. Here, running the first stage from the same B+ as the driver would help to keep quiescent current up. The HC voltage of most 6sn7s is 200V so should not be a problem. Also, what you'll lose in higher PS noise you'll gain in better rejection. These mods are simple and worthwhile evaluating seperately. I'd be interested in your feedback should you try any of them.FWIW, both stages are not very linear but the driver particularly so due to the high voltages it must swing (which would be helped considerably by mod 2), however distortion will be mostly 2ndH and I've found that this is actually desirable in SE driver stages since it goes a long way towards cancelling some of the distortion generated in the OP stage since it's opposite in phase.
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