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Chip Amps Amplifiers based on integrated circuits

LM1875 transconductance amp
LM1875 transconductance amp
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Old 7th May 2015, 08:59 AM   #21
kp93300 is offline kp93300  Malaysia
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Pawel. thanks for guidance.
My dc offset measure 30mv and 79 mv. Is this ok ?
I notice you mention 22k for the input pin to ground to lower output offset?
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Old 7th May 2015, 10:25 AM   #22
padamiecki is offline padamiecki  Poland
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Quote:
Originally Posted by kp93300 View Post
Pawel. thanks for guidance.
My dc offset measure 30mv and 79 mv. Is this ok ?
I notice you mention 22k for the input pin to ground to lower output offset?
what value res do you have at the input?
try to put 100k potentiometer as input load, set it for the lowest offset and solder resistor that value.

I got with 100k input res the offset +70mV DC
with 18k I got -13mV
so my guess that 22k would give the lowest offset..
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Old 7th May 2015, 01:26 PM   #23
thiagomogi is online now thiagomogi  Brazil
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Quote:
Originally Posted by kp93300 View Post
thanks thiagomogi for the suggestions .
What about raising the value of the resistor from speaker negative to ground? I have 0.5 R now.
No ... parallel with speaker ...... Take a look here .

https://www.passdiy.com/project/ampl...-range-drivers
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Old 7th May 2015, 01:31 PM   #24
thiagomogi is online now thiagomogi  Brazil
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quote from Nelson Pass

"Let's assume that we want to achieve optimal frequency response for a given system. This is not necessarily the best-sounding system possible but causes less subjective arguments. We can trim the damping "Q" of the low frequency roll-off knee by trying different values for R0 (L0 = 0 in this case) until it flattens out to our taste. Most of the drivers we worked with were happy with values from about 22 ohms to about 47 ohms. It is surprising that so little damping gave such dramatic results but as previously pointed out, this category of loudspeaker responds well to small amounts of damping"
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Old 7th May 2015, 01:55 PM   #25
forr is offline forr  France
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Quote:
Originally Posted by padamiecki View Post
I wrote about this, this amp has very high output impedance, so resonances in the speaker are not damped.
In the shown schematics, the output impedance is no more than 68 Ohm, the resistor value in parallel with the load.
Even with an infinite current source, the speaker main resonance is still somewhat mechanically damped.
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Old 8th May 2015, 04:01 AM   #26
kp93300 is offline kp93300  Malaysia
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I notice that the dc at the output depends on whether the speaker is connected
Without speaker connected, i measure about 1 v !
With speaker connected i get about 90 v and 110 mv .
This is still too high for my liking but i listen to the set for about 2 hrs already and the sound is very nice indeed,
I try changing the 12K input to ground resistor to 10K and 22 K and do not get much change in the dc offset at the output.
The amp is driven by a transistor buffer and the 100k volume control is in front of the buffer circuit.

Last edited by kp93300; 8th May 2015 at 04:03 AM. Reason: add info
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Old 8th May 2015, 10:00 AM   #27
arcgotic is offline arcgotic  Romania
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If i will drive this amp directly from a Xonar U7 usb card, that outputs 1 Vrms (2.828 Vp-p), how many Watts will it produce? Or should i add a voltage amp before LM1875?

I need the full 20W output capability..
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Old 8th May 2015, 10:16 AM   #28
padamiecki is offline padamiecki  Poland
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Quote:
Originally Posted by arcgotic View Post
If i will drive this amp directly from a Xonar U7 usb card, that outputs 1 Vrms (2.828 Vp-p), how many Watts will it produce? Or should i add a voltage amp before LM1875?

I need the full 20W output capability..
should last for 20W...
you will get 10A peak from 1V peak at the input when your speaker is 5 Ohm and current sensing resistor is 0.5 Ohm
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Old 12th May 2015, 02:55 PM   #29
kp93300 is offline kp93300  Malaysia
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I want to report that the high dc offset is due to a fake chip. After change dc offse is about 10mv
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Old 14th May 2015, 04:24 AM   #30
kp93300 is offline kp93300  Malaysia
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I notice there is a thump on switching off the power , probably due to uneven capacitor discharge in the V+ and V- rails .
Is a speaker relay the only option?
I prefer the sound of this amp over a gainclone build with Peter daniel board.
It sound louder than the specs suggest and the bass is more tuneful .
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