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Old 18th April 2011, 02:45 PM   #1
BigE is offline BigE  Canada
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Default Phase linear pre-driver emitter resistor

What colour are the 10 ohm resistors on the pre-drivers supposed to be?

All the others big ones in my amp are brown. The 10 ohms resistors are black. One was touching some silk insulation and regular insulated wire. The silk insulation is burnt, the regular is melted.

I suspect that these were originally brown, but overheated and changed colour.

Can anyone verify what colour they are supposed to be?

I suspect that changing them to 5W parts might be a good idea -- judging by size, they appear to be 2 watt parts.

Are they originally 2 watt parts? The schematic does not say.

Thanks.

Last edited by BigE; 18th April 2011 at 02:56 PM.
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Old 18th April 2011, 03:10 PM   #2
jaycee is offline jaycee  United Kingdom
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Colour wont be important, they probably were brown indicating carbon devices. I suspect if they were 2W and burnt out, there are further problems with the amp such as oscillation.

I'd use metal film devices if you are replacing them - also space them away from the pcb by 10mm if you can.
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Old 18th April 2011, 07:36 PM   #3
BigE is offline BigE  Canada
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Thanks Jaycee! Why metal film?

I tried 2x12 ohm shinkoh in the output zobel, and while detailed, the bass was soft and there was EMI from the SMPS in the digital gear on the scope -- made for a hazy background. Not a lot of depth either. Nice details, but boring.

I went back to the original carbons and the music came to life again.

I've replaced the other 0.22 ohm carbon emitter resistors with 5W mills wirewound. They sound fine.

I am a bit hesitant to use any kind of metal film. I'm considering either the 5W mills or the 2 or 5 W kiwame or 2 watt Amtrans here.

But I am most concerned of the power, since there was burnt insulation where a wire was touching the resistor body.
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Old 19th April 2011, 10:47 PM   #4
BigE is offline BigE  Canada
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I tried the mills as emitter resistors in the phase linear.

They sound far less lively than the stock carbons.

I can find only vishay/dale wire wound as an alternative. Are there any other 0.22R 2 watt + resistors that should be considered?

Caddock does not make 0.22R, only 0.2. and I believe 0.02 - that's an expensive proposition.
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Old 20th April 2011, 12:53 AM   #5
wg_ski is offline wg_ski  United States
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You're probably experiencing the effects of changing the quiescent bias more than anything else. If you just swap resistors out it's likely to change. It can be set anywhere from a full class B to a moderate AB and still work properly, and it changes the basic "sound" (and the stability or lack thereof).
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Old 20th April 2011, 06:21 PM   #6
BigE is offline BigE  Canada
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The quiescent bias does not change with a different *type* of emitter resistor. It measures exactly the same with either resistor type in place. I make sure after each and every change that the bias is exactly as it was.

The Mills are more detailed and rounder sounding than the carbons, which are inaccurate and diffuse, but... they have tone, life and ambience that is absent with the Mills. Transient attack is blunted, while with the original carbon it is not -- drum strike for eg.. Also reverb trails are much shorter with the Mills.

I can see why Mills would be popular in a studio environment. They separate instruments and layers really well, on a precisely defined stage. Smaller than the carbons, but very precise. OTOH, it could make for too much reverb.

Oh sure, it can be argued that the mills is showing what is there and there may be problems elsewhere in the chain, however, I don't think a resistor can add transient attack. But it can reduce HF, and LF response, which is what I hear.

As for the carbons, yes, very inaccurate, but pleasing.

What I am looking for is both accuracy and musicality.
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Old 20th April 2011, 06:40 PM   #7
wg_ski is offline wg_ski  United States
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Quote:
Originally Posted by BigE View Post
Oh sure, it can be argued that the mills is showing what is there and there may be problems elsewhere in the chain, however, I don't think a resistor can add transient attack. But it can reduce HF, and LF response, which is what I hear.
Like marginal stability. Not necessarily outright oscillation, but trying to - with an above-band gain peak and lots of ringing. This results in a bright, almost artificially detailed sound. I wouldn't trust a Phase Linear not to do this.The Q-point has a huge impact here, but there are other ways of doing it. Play with the value of the lead compensation capacitor and you'll see exactly what I mean. Some resistors are more inductive than others, and some (like carbon) increase in value radically at RF frequency and still have low inductance. This will tend to stabilize it. If this is what is happening, it is easily measureable.


Quote:
What I am looking for is both accuracy and musicality.
You may need to look at a different amp. The PL is a good compromise with what was available at the time, but they can be made better now.
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Old 20th April 2011, 09:37 PM   #8
BigE is offline BigE  Canada
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Quote:
Originally Posted by wg_ski View Post
Like marginal stability. Not necessarily outright oscillation, but trying to - with an above-band gain peak and lots of ringing. This results in a bright, almost artificially detailed sound.
This sounds like the Phase linear with the carbon resistors. There is a lot of ringing, especially with instruments like the vibraphone. Also, the space/ambience seems almost "echo-like". Like artificial space.

Quote:
Originally Posted by wg_ski View Post
I wouldn't trust a Phase Linear not to do this.The Q-point has a huge impact here, but there are other ways of doing it. Play with the value of the lead compensation capacitor and you'll see exactly what I mean. Some resistors are more inductive than others, and some (like carbon) increase in value radically at RF frequency and still have low inductance. This will tend to stabilize it. If this is what is happening, it is easily measureable.
I have a 'scope now. Can you please let me know how to measure this stability?

The mills wirewound non-inductive does not have the ringing and spaciousness of the carbon. Is it possible that the mills is stable and the carbon is not?

Quote:
Originally Posted by wg_ski View Post
You may need to look at a different amp. The PL is a good compromise with what was available at the time, but they can be made better now.
It is possible.
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Old 21st April 2011, 03:40 AM   #9
BigE is offline BigE  Canada
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Oops. my bad... I misunderstood..... it is the inductance of the wire wound causing the trouble. yes, it sounds like that.... The carbons sound fine.

Would metal film have the same issue?
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Old 21st April 2011, 06:13 PM   #10
wg_ski is offline wg_ski  United States
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MOX would be different, not necessarily better... or worse. They tend to be low-ish inductance, but rise in value significantly with temperature. You can't burn 'em out, partly because R skyrockets when you get them glowing orange. I would expect their HF behavior to be more similar to wirewound than carbon, however.

To see if there is anything really going on, look at the *small signal* square wave response, with the scope probe directly at the feedback pickoff point and normal loading of the amp. More overshoot or ringing means less stable. Use a 1k or so test signal and zoom way in on the leading or trailing edge. This should separate stability-related artifacts from slewing-induced ones.

Marginally stable emitter followers really hate inductance in the emitter leg. That's why zobels are used in the first place. The usual 'fix' for all this is to use base stoppers. Back in those days, the low beta made it impractical. If you've replaced the outputs with MJ15024's you can add them and see if it helps anything.

Last edited by wg_ski; 21st April 2011 at 06:16 PM.
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