Rod Elliot P3A build sounds too bright/harsh/agressive

Ravenash

Member
2016-03-22 1:00 am
Hi guys,

I have just finished building the P3A amplifier, following all the exceptional feedback on the web, and unfortunately the sound is below my expectations. I realize that this is because of some components are used, but I simply cannot identify them.

My main problem is that the sound is very bright, agressive. I am looking for a warm, natural sound. The amplifier at this point has excellent transparency of instruments, good control and is doing an excellent job with all the frequency range from deep, strong bass to crisp highs. HOWEVER, the sound is very "loud", very harsh, bright, agressive. It is very tiresome to listen to it. I think that the middle frequencies are being brought up in front too loud. ( I have noticed this behaviour on the powerful Pioneer A-878, the reason why I changed it for the very warm Onkyo 8670 ).

I have used the exact values for all components in the design. WIMA capacitors for the 100 pF, all resistors are 1% Welwyn, the finals are MJ21193/4, drivers BC139/140 and others BC546 as specified. All A/C capacitors are polypropylene. My main concern is C1. I have used a Vishay polyprop capacitor - 4.7uF at 150V.It is massive in size, and seems of very good quality, but I am worried it is the one giving me this agressive, screamy sound.

Can you please provide any info about how I can make the P3A sound warmer and more relaxing to listen to ?

( mention - I am using this without a preamplifier at the moment, directly from the sound card output, but I also do that with the Onkyo and the sound is much warmer - could a preamp solve this problem ? )

Thank you so much.
 

currentflow

Member
2008-04-15 11:30 am
My main problem is that the sound is very bright, agressive. I am looking for a warm, natural sound. The amplifier at this point has excellent transparency of instruments, good control and is doing an excellent job with all the frequency range from deep, strong bass to crisp highs. HOWEVER, the sound is very "loud", very harsh, bright, agressive. It is very tiresome to listen to it.
A harsh-sounding amplifier might be the result of instability. If you have an oscilloscope and a signal generator, this might help reveal the cause of the problem.
I have used the exact values for all components in the design. WIMA capacitors for the 100 pF, all resistors are 1% Welwyn, the finals are MJ21193/4, drivers BC139/140 and others BC546 as specified. All A/C capacitors are polypropylene. My main concern is C1. I have used a Vishay polyprop capacitor - 4.7uF at 150V.It is massive in size, and seems of very good quality, but I am worried it is the one giving me this agressive, screamy sound.
I think you meant to say BD139/140 (not BCxxx) drivers. For C1, why not try a different type of capacitor?
Can you please provide any info about how I can make the P3A sound warmer and more relaxing to listen to ?

( mention - I am using this without a preamplifier at the moment, directly from the sound card output, but I also do that with the Onkyo and the sound is much warmer - could a preamp solve this problem ? )
A well-designed amplifier should sound fine without a pre-amplifier, given sufficient line levels to drive the amplifier to its full output capability. The problem is more likely to be caused by instability.
 
This is a proven good sounding amp, so the issue must be in the implementation - or you have nasty speakers. I have only limited experience, but I would suggest looking for oscillations as mentioned already. After that I would say the most likely issue is feedback compensation capacitor choice and choice of other capacitors. Compensation capacitors - try Silver Mica or ceramic np0. Some metallized foil caps sound a bit harsh to me.
 

Ravenash

Member
2016-03-22 1:00 am
The Speakers are Pioneer DSS-9 , they should not be an issue. I will attach some pictures in a minute maybe you can see something wrong there

Link 1[IMGDEAD]https://s11.postimg.org/4zt2s2qfz/20160322_145028.jpg[/IMGDEAD]

Link 2[IMGDEAD]https://s11.postimg.org/is2w9yrtr/20160322_144926.jpg[/IMGDEAD]

Link 3[IMGDEAD]https://s10.postimg.org/kmd6br651/20160322_144913.jpg[/IMGDEAD]
 
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Try using a normal cap for C1 (assuming that is the big green cap floating in the air). By 'normal' I mean small, cheap, possibly lower value. If this solves the problem then you almost certainly had oscillation. The problem is not that the cap is defective, but simply that it is large and hanging over the circuit.
 

StephenR

Banned
2016-01-15 8:26 pm
Is this the schematic?
 

Attachments

  • P3A.JPG
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If it is not oscillating, having actual high frequency response might be bothering you. check on headphones versus the speakers.
Check for oscillating with a VOM on 20 vac scale with a 390 pf cap in series with the negative probe. Steady voltage with input shorted is ultrasonic oscillation. Or if scope equipped, look for fuzz on the waveforms. Oscillation is random in phase and jitters around so the trace fuzzes up.
I've only heard two types of speakers in my life that can do high frequency sources right. A pair of Altec Lansing Voice of the Theaters, ( in a theatre in 1967) and the pair of similar design ones in my tag line. Oh also the updated SP2 revision G in the store. It doesn't mean other speakers can't do it, but I've never heard any others that could in person. My test is, does piano source sound like a piano? Especially top octave piano.
high frequency intermodulation distortion is very annoying to me. Most speakers do that, and a lot of amplifiers. Try mixing 10 khz and 14 khz and see if the resultant is correct. Even in sim.
As far as source, my $80 Cd player and M97eraIV Mag phono cartridge sound about the same to me, except for pop and crackle on the vinyl surface. CD source is not inherently bad, IMHO. But my CD player is not a 1 bit converter that was all the rage 15 years after CD was invented. Warning my ears roll off at 14 khz, same as the speakers I bought. In 1967 before the Army ruined my ears at summer camp, my hearing went all the way up. My hearing is not as bad though as the deacons at church that think their all Y***** sound system with a Korg electric "piano" sounds good.
 
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Yes, the big cap is C1. I will buy a small one and see what happens. Thanks. I have been told that I do not have power supply decoupling. Can anyone please explain to me a bit how this would affect sound and how to fix ?
Actually you do have local power supply decoupling, that's what the two red electrolytics do. They don't look very big though, so you should still have 2x 2200 - 10000 µF in your power supply... and it looks like you do.

Hint: Twist power cables. The mess you have there right now may invite magnetic coupling to the amplifier and increases power supply source inductance.
 
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