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increase input sensitivity of power amp
john11
post Aug 4 2013, 07:02
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Hi and thanks for reading this post.

I have 2 power amps driving a pair of speakers via an active crossover.

The first amp drives the bass mid unit, the second amp drives the tweeters.

What i am finding is there is not enough treble on the music, probably because of the amp design, the sensitivity of both amps is different leading to one amp sounding louder than the other.

I have been fiddling around with it for the past few days and perhaps have come up with a solution. I want to raise the sensitivity of the amp driving the tweeters, there is a 200k ohm resistor across the phono input (screened cable input) of the amp driving the tweeters:

1. What if i were to remove this resistor all together?

2. there is a 20k ohm resistor and 5 microfarad capacitor in series with the phono inpiut, which then goes to the op-amp, what if i were to reduce this by 50 percent. Would i have to change the capacitor value as well.

3. The input impedance of the power amp needs to be high, but i read about impedance matching where i can match up the output impedance of the pre amp with the input impedance of the power amp and get good results. The output impedance of the pre amp is 100 ohms, so would it be okay to replace the 20k ohm resistor with one of 100 ohms value.

Many thanks.
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AndyH-ha
post Aug 4 2013, 07:56
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The input of a phono preamp is an important part of the circuit. It must load the phono cartridge correctly to get the proper frequency response. Probably a better idea is to change thing at the input or output of the power amp. A fixed value resistive drop (or better, an actual volume control circuit) on the input of the power amplifier you believe to be overly sensitive, to bring it in line with the other, would be the best solution -- if that really is the problem.
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john11
post Aug 4 2013, 08:24
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Hi. Thanks for the reply.

Made a mistake. The cap is 500 nano-farads, not 5 micro farads.

The amp in question is a power amp , not sure what you mean by "The input of a phono preamp is an important part of the circuit"

Are you advocating lowering the value of the 20k resistor, what is the lowest safe value do you think.

What about the 220k resistor across the phono input.

Many thanks.
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john11
post Aug 4 2013, 08:27
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Oops, sorry, rushed the last reply.

I am talking about any way to increase the sensitivity of the power amp which is driving the tweeters, don't really want to mess around with the other amp.

Thank you.
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AndyH-ha
post Aug 4 2013, 10:17
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You opened by writing about messing around with a "phono input". The only place I've ever seen that label is the input of a phonograph preamp. If that is not what you were trying to convey, I don't know what. A pure power amp just has a line level input, nothing else (it usually is in the form of a RCA phono jack, which is a different thing altogether). An integrated amp can have multiple inputs.

I don't think you can get anywhere with "increasing the sensitivity of a power amp" without some more serious adventures. The normal design is working in the linear part of it amplification potential and trying to get around that is likely to lead to distortion problems. The better solution, by far (if, as I already said, you are certain an amplifier difference is the cause of what you are experiencing with your music), is to decrease the input to the other power amplifier (i.e. turn down the volume) so its output is better balanced to the one you think you want to change.

As an experiment, you might want to temporarily switch places between the two power amplifiers.
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antz
post Aug 4 2013, 12:22
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Since you're saying there's not enough treble, we're probably talking about a several-dB increase needed in sensitivity. It would help to know more about the components of the system and what is interconnected to what, in terms of signal path. Where is the volume control, for instance? What is your input source?

It's very unlikely that the design of your amp will allow that sort of increase without running into serious problems. The changes you've suggested will not make sufficient difference. As Andy suggests, decreasing the sensitivity of the bass amp is more sensible and you'll then have to increase the overall volume setting to compensate. As an alternative, effectively the same thing would be to alter the active crossover to feed less bass.

If you really must increase the input to the treble amp, you're likely to have to insert some sort of "gain-block" pre-amp between the preceding stage (presumably the active crossover output) and the amp input. That would, of course, cause an increase in noise but with careful selection it should be acceptable.

This post has been edited by antz: Aug 4 2013, 12:24
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Arnold B. Kruege...
post Aug 4 2013, 13:17
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QUOTE (john11 @ Aug 4 2013, 02:02) *
Hi and thanks for reading this post.

I have 2 power amps driving a pair of speakers via an active crossover.

The first amp drives the bass mid unit, the second amp drives the tweeters.

What i am finding is there is not enough treble on the music, probably because of the amp design, the sensitivity of both amps is different leading to one amp sounding louder than the other.

I have been fiddling around with it for the past few days and perhaps have come up with a solution. I want to raise the sensitivity of the amp driving the tweeters, there is a 200k ohm resistor across the phono input (screened cable input) of the amp driving the tweeters:

1. What if i were to remove this resistor all together?


I presume that by phono input you mean RCA jack. If so the proposed change will do nothing or very little.

The resistor is a drain resistor, designed to bleed off charges on the input capacitor. If forms a voltage divider with the source impedance of the device driving the power amplifier. The source impedance of this device might be 200 ohms, which would cause a 0.01 dB loss. This is insignificant.

Increasing gain is more difficult than decreasing gain. If you want to balance the gain of two amplifiers, the easiest means is to decrease the gain of the amplifier with higher gain. A simple resistive divider or attenuator can do this.

QUOTE
2. there is a 20k ohm resistor and 5 microfarad capacitor in series with the phono inpiut, which then goes to the op-amp, what if i were to reduce this by 50 percent. Would i have to change the capacitor value as well.



Depending on how this is wired in conjunction with the 200K resistor mentioned above, we may have now found the source of a 1 dB loss or not. Same basic answer - not worth the trouble to change.

QUOTE
3. The input impedance of the power amp needs to be high, but i read about impedance matching where i can match up the output impedance of the pre amp with the input impedance of the power amp and get good results. The output impedance of the pre amp is 100 ohms, so would it be okay to replace the 20k ohm resistor with one of 100 ohms value.


Again, not worth the trouble.

Do the power amplifiers have their own gain controls?

If they do, then you can use them to balance the gains - apply the rule of decreasing the gain of the amplifier that is too loud.

If they don't then you can add an external gain control. In your situation I would find some 5K or 10K potentiometers and wire them up as variable attenuators.

You can buy external devices that put this in a nice little box with connectors.

Here's an example of one:

http://www.amazon.com/Sescom-SES-MKP-25-Pr...l/dp/B007G2PZSM
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Speedskater
post Aug 4 2013, 16:11
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It might be a active crossover problem. For a quick test, try swapping amplifiers.


--------------------
Kevin
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john11
post Aug 5 2013, 06:22
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Hi Thanks for the replies, much appreciated.

In the end i have cheated and removed the old tweeter and replaced it with a higher sensitivity one, also i have replaced the old speaker cable with the thickest silver cable i could find.

This has increased the treble and solved the sensitivity problem.

Many thanks for all your replies.

Just one last question, concerning the 20k resistor between the phono input and op-amp, twenty thousand ohms sounds like a lot of ohms, why does lowering this by 50 percent have such little effect.

Many thanks.
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DVDdoug
post Aug 5 2013, 19:10
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QUOTE
Just one last question, concerning the 20k resistor between the phono input and op-amp, twenty thousand ohms sounds like a lot of ohms, why does lowering this by 50 percent have such little effect.
The two resistors form a voltage divider, where the voltage (signal) is divided in proportion to the resistance. You are loosing about 10% of the signal, which is less than 1dB. If you cut the 20 k resistor in half, you will increase the signal to about 95% which is less than 1/2 dB increase.

QUOTE
1. What if i were to remove this resistor all together?
Then, the capacitor would be directly across the preamp's (or source's) output. That could potentially damage the preamp. There is even a slight possibility that the power amp would oscillate, burning-out the amp and/or the tweeter! (But if your preamp/source survives, you'll get about +1dB.)

QUOTE
...also i have replaced the old speaker cable with the thickest silver cable i could find.
That's unnessary... Unless you have 100 feet of wire or more, the wire resistance is very-low already (near-enough to zero-ohms) compared to a 4 or 8-Ohm speaker. So again, this will make a fraction of a dB difference at best. And if you want low resistance wire, it's al lot cheaper cheaper to use a lot more copper than to use a little more silver.... Just looking at Wikipedia, it appears that silver is about 6% more conductive than copper. It would be cheaper to use twice the copper and cut the resistance in half.
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