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Old 27th March 2006, 05:16 PM   #1
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Default Harsh sounding amp

Hi, am a DIY but not an expert in amplifier design, I've built this amp but the highs are sounding harsh and bass is not sufficiently present. I have measured the output impedance to be 0.437 R, but stilll don't have anyy clue on how to smooth the high frequencies. Any help would be welcome. Thanks
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Old 27th March 2006, 08:03 PM   #2
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Measure voltage drop on R1 and R2 to check out bias. Should be more than some 10mV on both if you want good sound. Furthermore you can add emitter resistors for Q3/4/5/10/11 in range of 33R to 220R to reduce transient intermodulation.

regards
Adam
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Old 28th March 2006, 07:31 AM   #3
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I don't see what R1 and R2 are for since there are onpy one output transistor per rail. You may try to remove them.

Use a trimpot instead of R11 in order to ragulate the idle current.

The schematic is very similar to Project 3A at Rod Elliot's site www.sound.au.com. Go there and read the description of the project.

Good luck.
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Old 28th March 2006, 10:00 AM   #4
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by sparkybg
I don't see what R1 and R2 are for since there are onpy one output transistor per rail. You may try to remove them.
Hi,

Under no circumstances should you remove R1 and R2,
they are essential for thermal stability.

The bias must be accurately set for low distortion.

see here :

http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm

edit : with feedback output impedance should be much lower than 0.4R.
I'd also add an output inductor with a 3.3R resistor in parallel for damping.

/sreten.
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Old 28th March 2006, 10:29 AM   #5
DarkOne is offline DarkOne  Slovakia
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I would add emiter restors to Q3, Q10, Q11. Values depends on currents you set for diff. and VAS stage. Resistor on Q3 should be lower value than on Q10 and 11.
Also I do NOT recomend removing rosistors R1&R2.

Regards
Robo
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Old 28th March 2006, 11:42 AM   #6
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Quote:
Originally posted by sreten


Hi,

Under no circumstances should you remove R1 and R2,
they are essential for thermal stability.

The bias must be accurately set for low distortion.

see here :

http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm

edit : with feedback output impedance should be much lower than 0.4R.
I'd also add an output inductor with a 3.3R resistor in parallel for damping.

/sreten.
UPS!!! Right! Do not remove them. A little lower values, down to 0.22ohm may be used, but they must be there.

Anyway I don't think they are the problem. Check the idle current. It must be at least 30mA per output device.

If you have access to oscilloscope you can check for high frequency oscillations. If the amplifier gets hot even when idle, this is often a symptom of oscillation. Adding emmiter resistors in VAS and DIFF stage may help a little with oscillations also I think.
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Old 28th March 2006, 12:13 PM   #7
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As already mentioned emitter resistors for Q3, 10, & 11, but also important is to use a capacitor across Q6 C to E!

Cheers Michael
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Old 28th March 2006, 04:26 PM   #8
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Michael
what capacitor do you suggest in this place?
I use 470nF/100V polypropylene. Do you think it is OK?

regards
Adam
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Old 28th March 2006, 05:07 PM   #9
PMA is offline PMA  Europe
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Default Re: Harsh sounding amp

Quote:
Originally posted by zeus_threat
Hi, am a DIY but not an expert in amplifier design, I've built this amp but the highs are sounding harsh and bass is not sufficiently present.
Any component of your audio chain could be responsible of harsh sound and I guess that any long-distance analysis would probably be useless.

Anyway - I would suspect wrong bias current and resulting crossover distortion. Try to measure voltage drop across end-stage emitter resistors. And if you happen to have test CD with 1kHz sinus, try it. You must not hear any sound from the tweeter, provided there is at least 12dB/oct high-pass.
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Old 28th March 2006, 05:07 PM   #10
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Hi Adam,

oh, you have also built the amplifier zeus_threath presented a schematic on?
How does it perform?

About the capacitor across a Vbe multiplyer, I can not for sure suggest exact value for the schematic presented here which would be ok because it depends on several things, I found that some leave it out while other use it, where some use the same value as you, others even much higher values, 10 uF and up.

I would say it depends on how much the load seen by VAS in following stage varies by signal and load and think it through in combination with trial n' error what would suit your ears, and eyes when looking at measuring results!
What I try to say is that the upper and lower half might not even necessary show up consistent load which becomes more crucial at higher frequencies in some cases.

BTW I agree also with you that input diff could be degenerated of course too.

Oh, and one another thing when I look closer at the schematic is b-e resistor for output transistors, I don't know what transistors are used for the outputs but one should find out if approx 3 mA is enough to pull off those transistors, as cross conduction comes to my mind.

Cheers Michael
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