2. amps-preamps
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increase power amp sensitivity

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 500 nano farad capacitor in series with the phono input, which then goes to the op-amp, what if i were to reduce this resistor 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.
goodsguys
heyraz
177 posts
 
Like Al said, there's not enough information in your post to make an informed suggestion. But I had another idea that may be a worthwhile alternative.

Why not simply attenuate the (louder) woofer amp before modifying the tweeter amp? Parts express sells attenuating plugs, I would try that before changing an amp's input. It's a cheap and reversible.
atmasphere's avatar
atmasphere
11,807 posts
 
As Heyra has said, its going to be a lot easier to reduce the input to the bass amp than trying to increase the gain for the high frequency amp!
heyraz
177 posts
 
If the attenuator plugs work out for you, once you roughly figure out how much attenuation you need, you could put a "passive preamp" (stepped attenuator switch) before the louder amp to fine tune the levels.
czarivey
5,946 posts
 
Phono Input is probably set-up for MM cartridge and you can change resistor value. The sensitivity of the phono input won't change.
Same may apply to line stage resistors.
Negative feedback resistor if lowered will bump the input sensitivity, but your amp may go unstable and scream at idle.
almarg's avatar
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almarg
9,670 posts
 
Phono Input is probably set-up for MM cartridge and you can change resistor value. The sensitivity of the phono input won't change.
I believe that the OP's reference to "phono input" was referring to the connector type (RCA), not to the signal type, and that a phono level signal is not involved. I suspect that reflects British terminology.
Negative feedback resistor if lowered will bump the input sensitivity, but your amp may go unstable and scream at idle.
Given that an op amp stage is being addressed, and assuming that it is not set up as a unity gain buffer, the gain of the stage will be raised if the value of the negative feedback resistor in that stage is raised, not lowered.

Good suggestion by Heyraz.

Regards,
-- Al
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