Hi folks,
Any chance that someone have the manual ?
I want to check if the bias is within specs. I have the service documents for the 8001, the layout of the amplifier board matches the 10004 boards 1:1, so I'am tempted to adjust for 5-5,5mV like recommended for the 8001.
appreciate your help,
best,
georg
Any chance that someone have the manual ?
I want to check if the bias is within specs. I have the service documents for the 8001, the layout of the amplifier board matches the 10004 boards 1:1, so I'am tempted to adjust for 5-5,5mV like recommended for the 8001.
appreciate your help,
best,
georg
Hi Siox,
Here is some advice I got from a friend, hopefully it helps!
Apply 1khz to input, adjust output to 20V rms, no load.
Measuring as usual across a positive and negative emmitter resistor on milivolt DC range, check and adjust the bias preset for 20mV or 10mV if can only access one of the emmitter resistors.
Here is some advice I got from a friend, hopefully it helps!
Apply 1khz to input, adjust output to 20V rms, no load.
Measuring as usual across a positive and negative emmitter resistor on milivolt DC range, check and adjust the bias preset for 20mV or 10mV if can only access one of the emmitter resistors.
Hi digitone,
thanks for your reply and the schematics.
can you ask for the details why this procedure makes sense in this context? and at which level (mV) the 1kHz should be applied to the input?
i already soldered some wire's for measuring on R124/R112 before i assembled the amp. 20mV seems quite high compared to the 8001. but i will check what the meter reads on all 4 channels now, to get a rough idea.
thanks for your reply and the schematics.
can you ask for the details why this procedure makes sense in this context? and at which level (mV) the 1kHz should be applied to the input?
i already soldered some wire's for measuring on R124/R112 before i assembled the amp. 20mV seems quite high compared to the 8001. but i will check what the meter reads on all 4 channels now, to get a rough idea.
I had no time to continue work on the amp... but if anyone else stumbles across this:
Crest does not have any documents left... they suggest:
"In general , ac current consumption should be monitored by amp probe and be fairly reasonable (1 to 2 Amps range) while at idle"
best,
georg
Crest does not have any documents left... they suggest:
"In general , ac current consumption should be monitored by amp probe and be fairly reasonable (1 to 2 Amps range) while at idle"
best,
georg
In fact i have most of the crest amplifier schematics. Lenny tedesco was the guy at Crest that had the knowledge of these amps, I have his email but haven't talked to him on 15years or so. hopefully he is still with us.
None of the schematics i have show what to set bias for. but i have this note in my files. I no longer remember if i got this from lenny or where it came from....
This is intended for units that have no manufacturer’s recommended bias set-up procedure. It is only a recommendation based on field-tested and other established recommendations. This procedure is intended only for standard bi-polar output stage amplifiers. It is not recommended for digital (switching) or FET (Field Effect Transistor) output stage type amplifiers that may have special procedures for bias adjustments.
Test equipment requirements:
● Stable, low-distortion sine wave oscillator capable of 1 volt RMS output at 20kHz.
● 20MHz oscilloscope.
● Distortion analyser.
● 8-ohm non-inductive load resistors of at least 50W power handling capacity. These will NOT handle the full output of most commercial amplifiers; they are for low level testing only. 500W or 1000W resistors are preferable. (The more power handling capacity they have, the better.) The non-inductive characteristic is a must for this adjustment.
● VariAC to establish recommended line voltage (100, 120 or 220/230/240VRMS depending on locale.)
Procedure:
Establish that the amplifier to be tested is in a stable operating condition.
Connect unit to load and sine wave oscillator and (if available) distortion analyzer. Set oscillator at zero output for the time being.
Apply line voltage and stabilize at recommended point for amplifier.
Set level controls (if present) to full level.
Apply 20kHz sine wave signal of sufficient level to amplifier to generate 2.828 to 5.656 V Peak to Peak on oscilloscope.
Adjust amplifier bias control until the crossover distortion is plainly evident in the oscilloscope display. (Bias is headed toward the “cut off” point of the output devices.)
Re-adjust the bias so that the crossover distortion just begins to disappear.
Go no further on the adjustment so as to prevent the amplifier from idling too hot and possibly being damaged by heavy signal levels. Reading the distortion on a distortion analyser is highly recommended in conjunction with the oscilloscope display.
If possible, lock bias controls with a dot of glyptal resin or ThreeBond adhesive.
Repeat for all channels that were repaired.
End of procedure.
None of the schematics i have show what to set bias for. but i have this note in my files. I no longer remember if i got this from lenny or where it came from....
General Bias Adjustment Procedure Recommendation.
This is intended for units that have no manufacturer’s recommended bias set-up procedure. It is only a recommendation based on field-tested and other established recommendations. This procedure is intended only for standard bi-polar output stage amplifiers. It is not recommended for digital (switching) or FET (Field Effect Transistor) output stage type amplifiers that may have special procedures for bias adjustments.
Test equipment requirements:
● Stable, low-distortion sine wave oscillator capable of 1 volt RMS output at 20kHz.
● 20MHz oscilloscope.
● Distortion analyser.
● 8-ohm non-inductive load resistors of at least 50W power handling capacity. These will NOT handle the full output of most commercial amplifiers; they are for low level testing only. 500W or 1000W resistors are preferable. (The more power handling capacity they have, the better.) The non-inductive characteristic is a must for this adjustment.
● VariAC to establish recommended line voltage (100, 120 or 220/230/240VRMS depending on locale.)
Procedure:
Establish that the amplifier to be tested is in a stable operating condition.
Connect unit to load and sine wave oscillator and (if available) distortion analyzer. Set oscillator at zero output for the time being.
Apply line voltage and stabilize at recommended point for amplifier.
Set level controls (if present) to full level.
Apply 20kHz sine wave signal of sufficient level to amplifier to generate 2.828 to 5.656 V Peak to Peak on oscilloscope.
Adjust amplifier bias control until the crossover distortion is plainly evident in the oscilloscope display. (Bias is headed toward the “cut off” point of the output devices.)
Re-adjust the bias so that the crossover distortion just begins to disappear.
Go no further on the adjustment so as to prevent the amplifier from idling too hot and possibly being damaged by heavy signal levels. Reading the distortion on a distortion analyser is highly recommended in conjunction with the oscilloscope display.
If possible, lock bias controls with a dot of glyptal resin or ThreeBond adhesive.
Repeat for all channels that were repaired.
End of procedure.
I normally use a bias of 3 mV across each emitter resistor for massively paralleled EF3’s. No signal, no load. That’s low by audiophile standards, but works for PA amps using EF3 output stages. It is high enough to be able to tell if you have a matching issue between output transistors (If they don’t bias to near the same voltage). It is too low for EF2, and can cause stability problems in massively paralleled QC stages. Much higher and it just wastes power. You can get lower crossover distortion with a higher bias (up to about 26 mV), but only if the circuit is refined enough to be able to tell the difference. A big high power class H amp usually is not so there is no point in running it higher.
The early versions of these Crest amps have two output transistor banks per channel. The upper bank is separately biased and has no adjustment. Use the emitter resistors on the LOWER bank to make the bias adjustment. Newer versions use mosfet switches for the upper bank so it’s harder to confuse where to make the measurement.
The early versions of these Crest amps have two output transistor banks per channel. The upper bank is separately biased and has no adjustment. Use the emitter resistors on the LOWER bank to make the bias adjustment. Newer versions use mosfet switches for the upper bank so it’s harder to confuse where to make the measurement.