Aleph-X builder's thread.

[Zen Mod]

OK - use that schematic and write down what voltages/currents are in important points

[SGregory]

Thanks for the link to the schematic. My start of this build was way back on post #16.
You need to know (measure) the equivalent resistance of R24 with R26 and V2. Then measure the voltage drop across them and for grins measure across D1. The current is calculated using OHM's law. D1 sets up the voltage reference for the CCS FET Q6 and should be different by a couple of volts. The difference in volts should be within range of the Vgs spec sheet for the IRF9610. While you are in the area, measure the voltage across R17 which should allow you to calculate the current through the zener D1.

You can also measure the current of Q5 and Q7 by measuring the voltage across R23 and R25. The latter gets a little complicated with the current sunk by Q9 and Q4.

[JBL4435]

Originally Posted by Zen Mod

OK - use that schematic and write down what voltages/currents are in important points

I'll do tomorrow, please check back.

In my amp:

R19/29 is 10k
McMillan is 2.2k
R12/24 is 680 ohms setting AC current gain to app 69%
I have 12 Fet's per Mono Block

[JBL4435]

Originally Posted by SGregory

Thanks for the link to the schematic. My start of this build was way back on post #16.
You need to know (measure) the equivalent resistance of R24 with R26 and V2. Then measure the voltage drop across them and for grins measure across D1. The current is calculated using OHM's law. D1 sets up the voltage reference for the CCS FET Q6 and should be different by a couple of volts. The difference in volts should be within range of the Vgs spec sheet for the IRF9610. While you are in the area, measure the voltage across R17 which should allow you to calculate the current through the zener D1.

You can also measure the current of Q5 and Q7 by measuring the voltage across R23 and R25. The latter gets a little complicated with the current sunk by Q9 and Q4.

OK, measured with the amp driven by a 1kHz signal? What input? Normal? High?

[Zen Mod]

just measure voltage across R23 and R25

that will show us current through Q6,Q5,Q7

Q9 and Q4 aren't conducting , hopefully , except as current limiters in clipping condition

[JBL4435]

Originally Posted by Zen Mod

just measure voltage across R23 and R25

that will show us current through Q6,Q5,Q7

Q9 and Q4 aren't conducting , hopefully , except as current limiters in clipping condition

OK, couldn't resist

R23/R25
4.76V and 4.78V means app. 0.012A

R17=10k
R24=330
R26=270
R20=221
V2=200 adjusted to ? (need to check)

[SGregory]

One thing to point out is to check the components value vs. actual. Although most of the errors have been me in the past grabbing the wrong part. I do have on my quality shame board two resistors marked as 330R that were actually 10k.

I will keep checking back, but will be gone for the next few days so I have not abandoned your effort. I am sure others like Zen Mod who are much better at this than me will chime in.

[Zen Mod]

Originally Posted by JBL4435

OK, couldn't resist

R23/R25
4.76V and 4.78V means app. 0.012A
....

if you're sure that you have there 390R and up - that's OK

Originally Posted by JBL4435

R17=10k
....

that gives 600uA through zener ; waaay too low

make that 1K5 ..... or even better make current through zenner in range of 5mA

counting Iz=(Upos - Uz)/R17

if PSU is +/-15V , then R17= (15-9)/5mA = 1K2

it's possible that PSU is sagging on high level of power and zenner is shutting down , with so big value of R17

[JBL4435]

Originally Posted by Zen Mod

that gives 600uA through zener ; waaay too low

make that 1K5 ..... or even better make current through zenner in range of 5mA

counting Iz=(Upos - Uz)/R17

if PSU is +/-15V , then R17= (15-9)/5mA = 1K2

it's possible that PSU is sagging on high level of power and zenner is shutting down , with so big value of R17

Wow!
I'll try this and report THANK YOU!

[Eric]

Looks like I'm a few days late to the party here, though you've received some great advice already!

A few things that stand out to me as potential things to check:

1) Obvious question, but I have to ask: are the fets well matched?

2) Another obvious thing (already asked by Zen Mod) is the measured value of resistors in the circuit vs. values indicated in the schematic. I learned long ago to measure, measure, measure each and every part. This stemmed from ordering 220ohm resistors and receive 220kohm resistors for a past project.

3) How do things behave if you re-set AC Current Gain back to "normal" (50%) ?

4) Bias current sounds very low for using 12 mosfets per mono. Your bias and rail voltage appear a bit low for that many fets. What is the actual power dissipation for EACH fet? Measure voltage drop across each source resistor, divide this by the value of the source resistor, and multiply by rail voltage. Ideally, each fet should be running in the 20-30w range. If your dissipation is much lower than this, perhaps you want to parallel fewer fets for the output stage?