I am attempting to calculate the bass rolloff of my power amp - the input impedance is 20k & there is a 2.2uF capacitor at the input.
The output stage of the preamp I use has a 10uF capacitor, followed by a 1M resistor to ground.
Do these two capacitors work in series? ie. do they in fact = 1.8uF, which would have a significant effect on the frequency response of the power amplifier.
The output stage of the preamp I use has a 10uF capacitor, followed by a 1M resistor to ground.
Do these two capacitors work in series? ie. do they in fact = 1.8uF, which would have a significant effect on the frequency response of the power amplifier.
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The two capacitors will be in series and they will result in an equivalent cap of 1.8uF as you have already mentioned. The -3db at lf will be at 1/(2*pi*R*C) = 4.4Hz . Should be OK !
Note that ( if you want to ) changing the caps will have to be done carefully. Not all electrolytic ( or even film for that matter !) sound the same. They may be similar but sometimes you do hear a significant difference. Better you just keep what you have, they must have selected them after checking them out.
The 1 meg is to tie one end of the preamp cap to ( 0 V )ground. Otherwise when you plug it into the amp with power on it would make a huge 'pop' on your speakers with the possibility of a disaster.
Note that ( if you want to ) changing the caps will have to be done carefully. Not all electrolytic ( or even film for that matter !) sound the same. They may be similar but sometimes you do hear a significant difference. Better you just keep what you have, they must have selected them after checking them out.
The 1 meg is to tie one end of the preamp cap to ( 0 V )ground. Otherwise when you plug it into the amp with power on it would make a huge 'pop' on your speakers with the possibility of a disaster.
Thanks - my main concern is that they're in series and the larger cap on the preamp side is definitely adequate. So I can take out the input cap on the power amp and trim the dc offset using the onboard trimpots on the power amp.
If you don't keep experimenting with different systems then doing this is fine. I've done it too and like the result. Also note that you need to switch on the preamp first and let it settle down before turning on the power amp. Then power amp off before turning pre amp off !
It depends on the input bias details. 1M is probably far too high as a bias resistor unless the input stage uses FETs. For stability the usual LTP should see roughly equal DC resistor values at both sides.lordearl said:
Ok so what am I looking for in the power amp circuit to check the bias?
I built the preamp, but not the power amp so I don't have a schematic. Does this pic help at all?
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There appears to be some confusion here. The 1 meg is only for the output cap of the power amp. The power amp most likely will have a 22K resistor at it's input . So the 1 meg and 22k will come in parallel with each other. That will result in a load of 21.5K to the preamp.
The one meg is not required if the amp is connected to the preamp always as the 22K will act as the bleeder resistor for the 10uF capacitor . The problem arises if the preamp has been powered up without connecting anything to it. So it's simply best to leave the 1 meg there.
The one meg is not required if the amp is connected to the preamp always as the 22K will act as the bleeder resistor for the 10uF capacitor . The problem arises if the preamp has been powered up without connecting anything to it. So it's simply best to leave the 1 meg there.
For the risk to the amplifier, I'd be considering losing the preamplifier's output cap rather the amplifier's input cap. That way you can be assured that bandwidth or DC on the input or preamp output won't be such an issue - particulary in a year or so when you forget about that amp having no blocking cap. 
Yes Ian is correct. Additionally the 2.2uF cap might possibly be better than the 10uF cap , especially if the 10uF is an electrolytic type. But then you can't always be sure of the difference in sound without comparing them in circuit !
Nice idea, but it's a tube preamp so I am not keen on having 150vdc in the rca cables betweeen the preamp and the power amp, could be very dangerous
Correct. With a tube preamp you better have a capacitor at the output unless everything is hard wired with no connectors used.
No confusion. I agree that there probably is a 22k resistor providing bias, but we don't know without seeing the circuit diagram. Hence I saidashok said:
DF96 said:
Personally, I would leave things exactly as they are.
It's good to know what we're dealing with.
I still wouldn't remove that input cap unless it's an absolute certainty the 2 units won't be separated or you bolt them together, because modifying the input in an attempt to match it to an oddball preamp means it will likely need to revert to the original design to work with say, a commercial solid state preamp. As far as I know, up-market preamps may have line outputs specified as low as 200R.
As DF96 may be advocating, if this valve preamp is more or less "standard" in it's output parameters, then you don't need to do anything, despite the opinions we read about 2 caps being so much worse than 1, superfluous etc.
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