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Motor Boating Caused by SS Preamp. Help!?
Motor Boating Caused by SS Preamp. Help!?
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Old 19th October 2019, 02:30 PM   #1
Robbie010 is offline Robbie010
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Default Motor Boating Caused by SS Preamp. Help!?

Hi all,

I have a strange issue occurring with my setup that I am struggling to get to the bottom of.

I recently collected a pair of old Heathkit MA12 tube monoblocks after having them overhauled and recapped etc. I immediately noticed an issue with one amplifier motor-boating.

To cut a long story short, after being returned for further repair, the issue persists but I have determined that the source cause of the motor-boating is my preamp, which is an Audiolab 8000A.

If I switch the monoblock amps on and just leave them sat for any period of time the motor-boating does not occur. However, if I switch the monoblock amps on, then switch on my preamp, the left channel monoblock immediately starts to motor-boat. Alternatively, if I switch on the preamp first, then the monoblocks the motorboating does not occur immediately, but will occur after a short period of time.

The only way to stop the motor-boating is to switch off the monoblock amp for a few seconds.

Can anyone suggest what interaction between the two amps could be causing this issue?

Thanks!
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Old 19th October 2019, 03:11 PM   #2
martyh is offline martyh  United States
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Location: Wisconsin
I would suggest comparing the values of the newly installed parts against the schematic. Mucking around with capacitor values is a great way to turn a stable amp into a marginally stable amp. I learned that lesson the hard and expensive way. Hopefully someone more qualified than me will respond to your question but in the meantime it will not hurt to check what you have against the original schematic.
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Old 19th October 2019, 03:19 PM   #3
Alan4411 is offline Alan4411  Wales
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Just for clarity:

If you swap the two MA12s round, does the 'motor boating' follow the MA12 or stay with the left hand output of the Audiolab?

Explain what you mean by 'motor boating'.

The MA12s take 15 / 20 seconds to warm up enough to produce any sound, so there will be a delay switching them on after the Audiolab.

Does the motor boating stop if either of the two MA12s are switched off? And does the motor boating return as the MA12 is powered back on? (Best not to switch off and straight back on by the way, with these amps always wait 20 or 30 seconds before switching back on...)

Circuit is near standard Mullard 5-10, .pdf enclosed.
Attached Files
File Type: pdf MA12_CCT.pdf (1.32 MB, 39 views)

Last edited by Alan4411; 19th October 2019 at 03:30 PM. Reason: circuit added
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Old 19th October 2019, 03:37 PM   #4
Chris Daly is offline Chris Daly  Australia
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Motor boating is usually indicative of positive feedback, indicating a unwanted feedback path from output to input. You would need to check in particular any part of the circuit with a large time constant such as inter stage capacitance, but also could be transformers or power supply capacitance.

A schematic is a must, or indeed a return to the manufacturer to repair ... properly.
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Old 19th October 2019, 03:41 PM   #5
petertub is offline petertub  Sweden
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Motor Boating Caused by SS Preamp. Help!?
Quote:
Originally Posted by Robbie010 View Post
Hi all,

I have a strange issue occurring with my setup that I am struggling to get to the bottom of.

I recently collected a pair of old Heathkit MA12 tube monoblocks after having them overhauled and recapped etc. I immediately noticed an issue with one amplifier motor-boating.

To cut a long story short, after being returned for further repair, the issue persists but I have determined that the source cause of the motor-boating is my preamp, which is an Audiolab 8000A.

If I switch the monoblock amps on and just leave them sat for any period of time the motor-boating does not occur. However, if I switch the monoblock amps on, then switch on my preamp, the left channel monoblock immediately starts to motor-boat. Alternatively, if I switch on the preamp first, then the monoblocks the motorboating does not occur immediately, but will occur after a short period of time.

The only way to stop the motor-boating is to switch off the monoblock amp for a few seconds.

Can anyone suggest what interaction between the two amps could be causing this issue?

Thanks!
One cause of low-frequency oscillation aka motorboating is too large capacitors in the signal way. As these are inside a feedback loop they can cause
instability.
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Old 20th October 2019, 04:49 AM   #6
grovergardner is offline grovergardner  United States
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First, check your Audiolab preamp output with the volume down and use a multimeter to see of the Audiolab is passing any DC voltage. Another thing is to try a different source, any audio device with adjustable volume connected to the monoblocks to see if they exhibit the same behavior.

It sounds like one amp is only marginally stable and any, especially low frequency, signal generated by the Audiolab is sending it into oscillation. Another possibility is that the preamp connection is introducing some stray capacitance to the input of the amps which would set them off. And/or the Audiolab is acting like a giant antenna and picking up stray RF which is setting off the amps.
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Old 20th October 2019, 02:31 PM   #7
DF96 is offline DF96  England
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"Overhaul" and "recap" is likely to mean that the circuit has been 'upgraded' by someone who doesn't fully understand what he is doing, so the LF loop stability has been degraded. Return all capacitor values to the factory original. Then see what happens.
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Old 20th October 2019, 03:46 PM   #8
grovergardner is offline grovergardner  United States
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Quote:
Originally Posted by DF96 View Post
"Overhaul" and "recap" is likely to mean that the circuit has been 'upgraded' by someone who doesn't fully understand what he is doing, so the LF loop stability has been degraded. Return all capacitor values to the factory original. Then see what happens.
That's another possibility, yes. ;-)
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Old 20th October 2019, 10:15 PM   #9
LJT is offline LJT  Norway
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Lots of good advice has already been given. My guess is that you will find the issue to be with your Audiolab 8000A

"If I switch the monoblock amps on and just leave them sat for any period of time the motor-boating does not occur. However, if I switch the monoblock amps on, then switch on my preamp, the left channel monoblock immediately starts to motor-boat. Alternatively, if I switch on the preamp first, then the monoblocks the motorboating does not occur immediately, but will occur after a short period of time."
The tech manual for the audiolab clearly states that pre-out is not muted and therefore should always be switched on before and switched of after the power-amp. If your amp stars serial number 89xxx you MAY have approximately 8 second mute on your pre-out. If you pre-out mutes when inserting headphones, your amp will also have this 8 second delay.

The MA12 input does not seem to have capacitive coupling. Personally, I would consider using this on tube amps connected to the audio lab. Especially so if the motorboating stay with the left channel output of the 8000A and the pre-amp out and power amp in of your 8000A has not been separated as per tech. manual. Depending on serial number of the unit, the implementation of pre-out / PA-in seem to differ slightly between serial numbers starting 87xxx and 89xxx.


The output of the 8000A line/tone stage has two 35v rated electrolytic to protect the power amp from DC. However the servo of the power-amp will superimpose DC on the input and may therefore also produce a certain amount of DC on the pre-out of the Audiolab.

The MA12 seem to have it's own volume control - did you try to connect the source direct? If you use the 8000A only as RIAA and source selector, you could try to connect to the tape out and use MA12 volume control.
The Audiolab RIAA seem to have electrolytic blocking capacitor rated at 50V
Attached Images
File Type: png pre-out_power-in.PNG (52.7 KB, 166 views)
File Type: png Audiolab_8000a_servo.PNG (55.0 KB, 167 views)
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Old 20th October 2019, 11:14 PM   #10
cnpope is offline cnpope  United States
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Quote:
Originally Posted by LJT View Post
Lots of good advice has already been given. My guess is that you will find the issue to be with your Audiolab 8000A
If the OP could answer Alan4411's question "If you swap the two MA12s round, does the 'motor boating' follow the MA12 or stay with the left hand output of the Audiolab?", it might help to remove some of the guesswork.
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