DIY Audio Projects Forum

Subwoofer Amp Auto On Bypass or Mod?
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Author:  Synthsayer [ 24 Feb 2012, 19:03 ]
Post subject:  Subwoofer Amp Auto On Bypass or Mod?

The 200W subwoofer plate amp I have only turns on when the subwoofer output level of my Harman AVR 154 is set to max +10. Sometimes that does not turn it on.
I listen at low volumes mostly, and the sub amp will only turn on if I have the output level at +10 and turn the main volume up to where the system is playing very loud.
When I turn the volume back down, the amp goes off after about 20 minutes as it is designed to do.
Schematic diagram attached.
MCM 200W Subwoofer Plat Amp Schematic.pdf

Thanks in advance for your help.
Have a nice weekend, everybody.

Author:  SteveB [ 03 Mar 2012, 12:19 ]
Post subject:  Re: Subwoofer Amp Auto On Bypass or Mod?

The level detection and time delays are set by U202. I'm not familiar with the 2072D but looking at the datasheet you should be able to force it on permanently with a 1k resistor from U202 pin 5 to 4 or across C214 if that's easier. The idea being to stop the timing capacitor C214 charging up (the 20 minute delay to OFF).

Author:  Synthsayer [ 03 Mar 2012, 20:08 ]
Post subject:  Re: Subwoofer Amp Auto On Bypass or Mod?

You don't know how much I appreciate your help with this problem.
Thank you very, very much.
Have a great weekend!

Author:  Synthsayer [ 07 Mar 2012, 18:19 ]
Post subject:  Re: Subwoofer Amp Auto On Bypass or Mod?

@ SteveB
Your mod worked perfectly! Thank you, again!
This week I have been busy with work and helping my father with doctor visits.
Today, I found some 1k Ohm resistors and went to work.
I used the first mod you suggested of jumping between pins 4 and 5 of the 2072D with the 1k resistor.
I left the AVR off and turned the sub amp on with no input signal but connected to the driver and AVR sub outputs.
The sub amp turned on and has been on ever since then.
Yeah! Yeah! Yeah!
Thank you so much for helping me with this mod, again.
Forum members like you are the best!
I have posted on about 4 or 5 other forums for the last 3 - 4 months and you are the first person to give me knowledgeable, easy to follow steps for the mod.
Other people have done their best to help in different ways and their help is also greatly appreciated; I don't mean to minimize their input in any way.
Thanks again SteveB!
Have a great week and best of luck to you in everything you do.
Danville, KY

Author:  Synthsayer [ 07 Mar 2012, 18:28 ]
Post subject:  Re: Subwoofer Amp Auto On Bypass or Mod?

Greetings and a HUGE Thank You, to DIY Audio Projects Forums.
My sub amp stays on and is working great thanks to help from member steveB!
What a great day!
I returned 2 amps and almost set another one on fire trying to do the mod. Without help from this forum I would be stuck with an amp that was pretty much useless.
Many thanks to stevB and DIY Audio Projects Forums! :up: :up: :up:

Author:  SteveB [ 10 Mar 2012, 17:34 ]
Post subject:  Re: Subwoofer Amp Auto On Bypass or Mod?

Glad to have helped, it only took me a few minutes to work it out as a 'magic chip' delay timer had been used. Out of interest I Googled "Synthsayer sub" and yes you've definitely been looking for help for some months now! Why don't manufacturers think about the real world issues like you've had, all it needed was a delay timer bypass switch to make it optional.

Author:  Synthsayer [ 11 Mar 2012, 18:08 ]
Post subject:  Re: Subwoofer Amp Auto On Bypass or Mod?

Thanks again for your help. The sub is working great!
Yes, I did try to get some help for a while.
I agree with you about a bypass switch for the auto on. Great idea!
Have a great weekend and thanks again.

Author:  Synthsayer [ 15 Apr 2012, 11:53 ]
Post subject:  Re: Subwoofer Amp Auto On Bypass or Mod?

After over a month since performing this mod the sub amp has stayed on just like I wanted it to.

If you have one of these amps with the same problem, it would be a good idea to try this easy fix.

This is a great sub amp.

Author:  ariheikki [ 11 Sep 2013, 10:19 ]
Post subject:  Re: Subwoofer Amp Auto On Bypass or Mod?

hi,im having same problem with my jbl HTPS300 sub,i tried to upload service manual to attachment but upload failes between 20-30% and whole browser stuck and falls.can u help me too please(im new member)

Author:  Synthsayer [ 13 Sep 2013, 09:29 ]
Post subject:  Re: Subwoofer Amp Auto On Bypass or Mod?

Hi :-)
I found the manual for your JBL sub. Looks like a nice piece of gear.
I copied the following section from Troubleshooting portion of JBL manual:
If there is no output from
the subwoofer, check the
• System is on and a source
is playing.
• The HT PS300 is plugged in
and “Power” is on.
• If you are using the
THX input, make sure the
THX/ Normal switch is set
to “THX.”
• If you are not using the
THX input, make sure the
THX/ Normal switch is set
to “Normal.”
• If you are using the “High
Level Inputs,” check all wires
and connections between the
receiver/amplifier and the
subwoofer. Make sure all wires
are connected and that none
of the speaker wires are
frayed, cut, or punctured.
• Adjust the “Output Level”
• Review proper operation of
your receiver/amplifier.

All of that is pretty standard troubleshooting stuff.
You might try adjusting/increasing the Sub Output Level on your receiver.
Are you using an AVR?
If yes, which make?
I use a Harman AVR154. Some people on this and other forums told me that Harman AVRs have a weaker Sub Output?
Not sure if that is true.
This is just an example of how Sub Amps can react differently when hooked up to different signal sources, i.e. Pre-amps, AVRS, studio gear.
I have 2 MCM 200W Plate Sub amps as indicated in previous posts. I also have an O-Audio BASH 500W Sub Amp.
Neither of the Sub amp models I have would work with their built in Auto On circuits when hooked up to the AVR154. I tried turning up the Sub Output Level on the AVR with no improvement :-(
In a small home studio, I use a consumer/pro type Tascam TM-D1000 digital mixer hooked up to a stereo power amp and one of the MCM 200W sub amps.
When hooked up to the Tascam mixer, the Auto On circuit of ALL the sub amps works properly???
No kidding.
Hook any of them up to the AVR and none of them will turn on with the Auto On circuit!
If you can find a JBL service manual for your sub and isolate the resistors that control the voltage on the Auto On section, you might be able to get it to turn on and stay on the way I did with the MCM sub amps.
Some of the other guys on the forum here are really great at finding fixes for these kind of problems.
I wish I could be more help.
Have a great weekend and good luck with the sub.
It looks like it will be really nice once you get it going :-)

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