30V dc Turn On Thump Forte FT1 Amp

john65b · 19th June 2009, 05:06 AM

Hey all,

Got a Forte FT1 5 channel amp that has a monster turn on thump that drops 30VDC/20VDC to the right/left speakers when turned on. After the thump it drops to less than 20mv DC offset on both channels.

I put in a Soft Start that slows inrush and delays the cap charge, but doesn't really help a lot. Just seems to delay the thump a little.

When turned off it races to 3V, and has a little turn off thump. I looked for bleeder resistors and there was none, so I put 5k ohm/5 Watt across each rail (there are dual primaries and dual secondaries , so two pairs of resistors or four total) and seems to help a little...

Amp sounds just fine after the annoying thumps. Previous owner just left it on all the time.

There is NO info on this amp online. Anyone have any experience with this amp? Its annoying as hell, and my speakers cant take much more...

h_a · 19th June 2009, 09:48 AM

Turn on thumbs are often caused by aged caps that charge differently fast, replacing them with new ones may help you.

Softstart and 5k resistors (4mA is much less than the bias current anyway) will not really help you with that.

Have fun, Hannes

Twaksak · 19th June 2009, 02:09 PM

Quote:

Got a Forte FT1 5 channel amp that has a monster turn on thump that drops 30VDC/20VDC to the right/left speakers when turned on. After the thump it drops to less than 20mv DC offset on both channels.

Speaker protect circuit will allow your voltage from startup to voltage stabalise and only then connect the speaker.

Chris

john65b · 19th June 2009, 03:56 PM

The speaker delay circuit would not really be feasible, as there are five channels, and not much room in the chassis.

It would be nice to find out if the caps are shot before I disassemble the amp to replace them...there are four 1000uf/63V on power supply.

If the PS caps were bad, wouldn't I see some nasty DC offset, or hum, or other audio issues along with the thump? It works perfectly fine after the monster thump on and slight thump off.

john65b · 19th June 2009, 04:01 PM

I do notice when shutting down one channel continues playing for 3 or so seconds longer that the other channel....maybe there are cap issues after all...

NOTE - Actually, there are two separate power supplies, one for the front and center (125 wpc each) and one for the rear channels (55 wpc each) and I had the center speaker and one of the rear channels connected, so thats why the different draw downs...

When fronts or center speaker outs are connected, no difference in draw down, and same in the rears...

john65b · 20th June 2009, 02:12 AM

OK, scratch what I had posted, as there IS a difference in draw down on shut down (and the pop at start up too) between the outputs by a couple of seconds...

Can the thump be because of a start up voltage imbalance between the two rails?

Trevor White · 20th June 2009, 05:14 AM

Quote:

Originally posted by john65b
OK, scratch what I had posted, as there IS a difference in draw down on shut down (and the pop at start up too) between the outputs by a couple of seconds...

Can the thump be because of a start up voltage imbalance between the two rails?

It's a poor design. It should have relays. What happens if one of the output transistors short out ?? Could even cause a fire in the speakers

But then again the unit could be faulty.

What you could do is to build an outboard speaker protection and dethump unit and use the 12 volt trigger from the HT unit to switch it on and off.

regards
trev

john65b · 23rd June 2009, 02:33 AM

OK, so I am convinced that the previous owner before my friend had the amp disabled the mute circuit. He did a pretty sloppy job replacing the caps (under the board) and ran a separate wire from the PS rails to the main channel output devices.

What is annoying is there is NO INFO on this amp anywhere. If anyone has a schematic it would be nice to send it to me...

Anyway, here is a pic (extra high res) of the circuit.

I have circled what appears to be a resistor or component missing that disables the mute circuit. The switch that disables signal-less channels leading to the circuit in the middle also does not function and may be a part of the mute circuit.

Anyway, any advice? I am contemplating just soldering up a timer relay an speaker outs, but would like to see if I could get the amp back to original first...like I said, the DC offset s perfectly low and stable after 5 seconds on all 5 channels, just pops like a SOB when turned on (30VDC spike)...All 5 channels all have this turn on thump.

john65b · 24th June 2009, 01:06 AM

Sorry for the unavailable pics...my server went down.

She is back up now...any advice??

john65b · 28th June 2009, 10:25 PM

Ha!. Found the issue. It was a resistor that was burned up - clearly see it in the pics. Replaced it and she is all good now. I looked at that resistor and thought it couldn't be that simple..

Thanks for the help...

19th June 2009, 05:06 AM	#1
john65b diyAudio Member Join Date: Jan 2005 Location: Chicago	30V dc Turn On Thump Forte FT1 Amp Hey all, Got a Forte FT1 5 channel amp that has a monster turn on thump that drops 30VDC/20VDC to the right/left speakers when turned on. After the thump it drops to less than 20mv DC offset on both channels. I put in a Soft Start that slows inrush and delays the cap charge, but doesn't really help a lot. Just seems to delay the thump a little. When turned off it races to 3V, and has a little turn off thump. I looked for bleeder resistors and there was none, so I put 5k ohm/5 Watt across each rail (there are dual primaries and dual secondaries , so two pairs of resistors or four total) and seems to help a little... Amp sounds just fine after the annoying thumps. Previous owner just left it on all the time. There is NO info on this amp online. Anyone have any experience with this amp? Its annoying as hell, and my speakers cant take much more... __________________ "I speak Spanish to God, Italian to Women, French to Men, and German to my Horse" - Charles V

19th June 2009, 09:48 AM	#2
h_a diyAudio Member Join Date: Sep 2006 Location: Graz, Austria	Turn on thumbs are often caused by aged caps that charge differently fast, replacing them with new ones may help you. Softstart and 5k resistors (4mA is much less than the bias current anyway) will not really help you with that. Have fun, Hannes __________________ fresh matched IRFP240/IRFP9240 fets \|\| AlephJ/JX-kits F5 transistor kits \|\| Burning Amp BA-1/2/3 transistor kits

19th June 2009, 03:56 PM	#4
john65b diyAudio Member Join Date: Jan 2005 Location: Chicago	The speaker delay circuit would not really be feasible, as there are five channels, and not much room in the chassis. It would be nice to find out if the caps are shot before I disassemble the amp to replace them...there are four 1000uf/63V on power supply. If the PS caps were bad, wouldn't I see some nasty DC offset, or hum, or other audio issues along with the thump? It works perfectly fine after the monster thump on and slight thump off. __________________ "I speak Spanish to God, Italian to Women, French to Men, and German to my Horse" - Charles V

19th June 2009, 04:01 PM	#5
john65b diyAudio Member Join Date: Jan 2005 Location: Chicago	I do notice when shutting down one channel continues playing for 3 or so seconds longer that the other channel....maybe there are cap issues after all... NOTE - Actually, there are two separate power supplies, one for the front and center (125 wpc each) and one for the rear channels (55 wpc each) and I had the center speaker and one of the rear channels connected, so thats why the different draw downs... When fronts or center speaker outs are connected, no difference in draw down, and same in the rears... __________________ "I speak Spanish to God, Italian to Women, French to Men, and German to my Horse" - Charles V

20th June 2009, 02:12 AM	#6
john65b diyAudio Member Join Date: Jan 2005 Location: Chicago	OK, scratch what I had posted, as there IS a difference in draw down on shut down (and the pop at start up too) between the outputs by a couple of seconds... Can the thump be because of a start up voltage imbalance between the two rails? __________________ "I speak Spanish to God, Italian to Women, French to Men, and German to my Horse" - Charles V


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23rd June 2009, 02:33 AM	#8
john65b diyAudio Member Join Date: Jan 2005 Location: Chicago	OK, so I am convinced that the previous owner before my friend had the amp disabled the mute circuit. He did a pretty sloppy job replacing the caps (under the board) and ran a separate wire from the PS rails to the main channel output devices. What is annoying is there is NO INFO on this amp anywhere. If anyone has a schematic it would be nice to send it to me... Anyway, here is a pic (extra high res) of the circuit. I have circled what appears to be a resistor or component missing that disables the mute circuit. The switch that disables signal-less channels leading to the circuit in the middle also does not function and may be a part of the mute circuit. Anyway, any advice? I am contemplating just soldering up a timer relay an speaker outs, but would like to see if I could get the amp back to original first...like I said, the DC offset s perfectly low and stable after 5 seconds on all 5 channels, just pops like a SOB when turned on (30VDC spike)...All 5 channels all have this turn on thump. __________________ "I speak Spanish to God, Italian to Women, French to Men, and German to my Horse" - Charles V

24th June 2009, 01:06 AM	#9
john65b diyAudio Member Join Date: Jan 2005 Location: Chicago	Sorry for the unavailable pics...my server went down. She is back up now...any advice?? __________________ "I speak Spanish to God, Italian to Women, French to Men, and German to my Horse" - Charles V

28th June 2009, 10:25 PM	#10
john65b diyAudio Member Join Date: Jan 2005 Location: Chicago	Ha!. Found the issue. It was a resistor that was burned up - clearly see it in the pics. Replaced it and she is all good now. I looked at that resistor and thought it couldn't be that simple.. Thanks for the help... __________________ "I speak Spanish to God, Italian to Women, French to Men, and German to my Horse" - Charles V

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