The currents. If I recall, my brother (who has a 555-II) measured it and found one running at 2x the current of the other (and, on both channels, so this wasn't due to some defect). I'll have to verify this and ask if it had any effect on the offset...
Sonically I'm pretty sure it made a difference because we concurred on the results...
Cheers!
Sonically I'm pretty sure it made a difference because we concurred on the results...
Cheers!
Hi Arif,
I was going to sit here to see where this went.
Do you have a diagram of a 555II you could send me? From memory the diff pair should be balanced (I think that's where you are at). I normally balance them. The circuit is balanced on the 555. Now, the rest of the circuit could unbalance the diff pair as it corrects for a large imbalance (fixing the DC offset). One would, in this case, measure the two base voltages wrt ground. This tells you if the unbalance is circuit or gain related.
A 2X difference is pretty large as far as I'm concerned. Yes, this directly is affected by the DC offset, that little op amp will work hard to correct the output DC potential and throw off the balance. I've always found the amps sound better when the diff pair is "happy".
-Chris
I was going to sit here to see where this went.
Do you have a diagram of a 555II you could send me? From memory the diff pair should be balanced (I think that's where you are at). I normally balance them. The circuit is balanced on the 555. Now, the rest of the circuit could unbalance the diff pair as it corrects for a large imbalance (fixing the DC offset). One would, in this case, measure the two base voltages wrt ground. This tells you if the unbalance is circuit or gain related.
A 2X difference is pretty large as far as I'm concerned. Yes, this directly is affected by the DC offset, that little op amp will work hard to correct the output DC potential and throw off the balance. I've always found the amps sound better when the diff pair is "happy".
-Chris
Well, just finished a conversation with my brother, confirming what I mentioned - he has a 555II, which does use an op-amp to servo the DC output to zero. Found the dif-amp running with unequal currents of about 1:2; this was corrected (I didn't ask how, but I would think by changing the collector resistor); DC servo took care of any <change in> offset that may have resulted.
Incidentally, he did mention that the differences between the 555 and 555-II include the DC servo and an additional pre-driver set of transistors in the 'II'.
Oops, but this is getting to be off-topic... Apologies there!!
Cheers
Incidentally, he did mention that the differences between the 555 and 555-II include the DC servo and an additional pre-driver set of transistors in the 'II'.
Oops, but this is getting to be off-topic... Apologies there!!
Cheers
K-amps,
No, I'm not saying that the dif-amp pair was disbalanced to produce a bad-sounding mid and HF. I'm saying that in the 555-II of my brother, this was the case (imbalance), so the only thing I can conclude is that my brother and I prefer the sound when its balanced!
Now somewhere around the forum there's been tons of discussion on 'euphonic distortion', and I can't help think about that...
OTOH Anatech says its supposed to be balanced, so maybe the 555-II we have was a bit bad....
Cheers
No, I'm not saying that the dif-amp pair was disbalanced to produce a bad-sounding mid and HF. I'm saying that in the 555-II of my brother, this was the case (imbalance), so the only thing I can conclude is that my brother and I prefer the sound when its balanced!
Now somewhere around the forum there's been tons of discussion on 'euphonic distortion', and I can't help think about that...
OTOH Anatech says its supposed to be balanced, so maybe the 555-II we have was a bit bad....
Cheers
Put them back!
Clem_o,
The collector resistors do not have much effect on tail current. The other collector from the diff pair signals clipping. That's all. Changing that resistor changes the sensitivity of the clipping detection circuit. So, the little light comes on at a different level. I strongly suggest the original part be reinstalled.
The only way to measure the current balance in that circuit is to interrupt the collector circuits and add a small resistor to measure across. You could use a real expensive hall type current probe. The errors would exceed the resistor method.
All anyone can really do is match the diff pair, and the other transistors in the circuit. The amp sounds really good when that is done.
-Chris
Clem_o,
The collector resistors do not have much effect on tail current. The other collector from the diff pair signals clipping. That's all. Changing that resistor changes the sensitivity of the clipping detection circuit. So, the little light comes on at a different level. I strongly suggest the original part be reinstalled.
The only way to measure the current balance in that circuit is to interrupt the collector circuits and add a small resistor to measure across. You could use a real expensive hall type current probe. The errors would exceed the resistor method.
All anyone can really do is match the diff pair, and the other transistors in the circuit. The amp sounds really good when that is done.
-Chris
Thanks for looking into this guys, but the amp isn't with me. :-(
As per discussion with my brother last night, he did mention that the '2nd dif amp transistor is connected to the clip indicator' - so I know he's aware of that; he also mentioned that the 1st dif amp transistor is running with about 1.2mA and the 2nd at about 2.5mA.
I'd really appreciate a schematic though, to further analysis on the amp! Arif or Chris, if it wouldn't be of too much effort could you send please: cong_alt01zzz@yahoo.com, just remove the three zzz's!
Much thanks,
Clem
As per discussion with my brother last night, he did mention that the '2nd dif amp transistor is connected to the clip indicator' - so I know he's aware of that; he also mentioned that the 1st dif amp transistor is running with about 1.2mA and the 2nd at about 2.5mA.
I'd really appreciate a schematic though, to further analysis on the amp! Arif or Chris, if it wouldn't be of too much effort could you send please: cong_alt01zzz@yahoo.com, just remove the three zzz's!
Much thanks,
Clem
By the way off topic, but I just reduced the nfb of my 545 and it sounds much better in the mids. Less glare and bite, more open, detailed and liquid.
I am not sure in dB how much I reduced but the NFB resistor common in 5x5 amps is 22.1k. I increased it to 33k. Also changed the input series resistor from 1k to 8k to compensate for added gain of reducing NFB. I left the input impedance setting resistance (input to ground) at stock, i.e. 22k.
I measured a 1k sinewave voltage before and after and I get about the same voltage gain before and after the mod.
Offset is the same and adjusted for Bias. I slightly overbiased it and I think, it sounds better. Heatsinks equilibrium at around 39C after an hour at idle. Bias at 20.6mV across each 0.47ohm emitter resister at equilibrium, up from the stock of 12mV (Chris can u confirm the stock voltage drop?, note this has 0.47E instead of the usual 0.22E found in the Mk.II)
I could have increased the input impedance resistance but figured keep it high... some preamps dont like it less than 25k.
Thoughts comments?
I am not sure in dB how much I reduced but the NFB resistor common in 5x5 amps is 22.1k. I increased it to 33k. Also changed the input series resistor from 1k to 8k to compensate for added gain of reducing NFB. I left the input impedance setting resistance (input to ground) at stock, i.e. 22k.
I measured a 1k sinewave voltage before and after and I get about the same voltage gain before and after the mod.
Offset is the same and adjusted for Bias. I slightly overbiased it and I think, it sounds better. Heatsinks equilibrium at around 39C after an hour at idle. Bias at 20.6mV across each 0.47ohm emitter resister at equilibrium, up from the stock of 12mV (Chris can u confirm the stock voltage drop?, note this has 0.47E instead of the usual 0.22E found in the Mk.II)
I could have increased the input impedance resistance but figured keep it high... some preamps dont like it less than 25k.
Thoughts comments?
Hi Arif,
You did increase the input impedance a little bit (23K to 30K) so your preamp might be happier. Your attenuation change is close to the reduction in feedback. Feedback ratio is easily calculated, but since I feel pretty dizzy right now maybe some else can do it for you.
My experiments with feedback echo what you see there. As you add feedback there comes a point where things stop improving, then the music starts to lose it's "life". So it's a balance between distortion, control (damping factor?) and dynamics. Most often you end up with far too much gain, so you look for a spot to bleed it off.
Your current bias (get it? current bias !
) is sitting at around 45 mA. I can't find my manual so I do not no what the recommended setting is. Man, I really miss all my service manuals. I do not trust my memory with that one.
-Chris
Edit: wrong word corrected
You did increase the input impedance a little bit (23K to 30K) so your preamp might be happier. Your attenuation change is close to the reduction in feedback. Feedback ratio is easily calculated, but since I feel pretty dizzy right now maybe some else can do it for you.
My experiments with feedback echo what you see there. As you add feedback there comes a point where things stop improving, then the music starts to lose it's "life". So it's a balance between distortion, control (damping factor?) and dynamics. Most often you end up with far too much gain, so you look for a spot to bleed it off.
Your current bias (get it? current bias !
) is sitting at around 45 mA. I can't find my manual so I do not no what the recommended setting is. Man, I really miss all my service manuals. I do not trust my memory with that one.-Chris
Edit: wrong word corrected
I dont want to go off a tanget here.... but what the heck, for class-AB amps, I remember NP saying 43-45mA is good. This was in reference to his Forte 200wpc amp (which is a similar design to the 5X5 amps).
On the flip side, Slone and Self say that the BJT EF stage is affected more by voltage drop (in the range of 47mV) rather than current drop. (MOSFETS are critical to current drop).
However If increase V drop to 47mV, I will cook the OP stage...
therefore perhaps to keep the OP stage current the same while doubling V drop, I may need to change the ER's from 0.47 to 1 ohm. I know DF will be half, but this amp will drive mids ina Tri-amp set up so I think that may be the way to go...
for class-AB amps, I remember NP saying 43-45mA is good. This was in reference to his Forte 200wpc amp (which is a similar design to the 5X5 amps).On the flip side, Slone and Self say that the BJT EF stage is affected more by voltage drop (in the range of 47mV) rather than current drop. (MOSFETS are critical to current drop).
However If increase V drop to 47mV, I will cook the OP stage...
therefore perhaps to keep the OP stage current the same while doubling V drop, I may need to change the ER's from 0.47 to 1 ohm. I know DF will be half, but this amp will drive mids ina Tri-amp set up so I think that may be the way to go...
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