Hi Peter,
I'm having trouble seeing the detail on the schematic regarding the potentiometers. One will certainly be to set the output transistor bias current an is very important. If you could scan a smaller area of the schematic and post it or email it to me I'll try and explain what each pot is for. For the bias current you probably want about 30mA to 50mA per transistor - or 120mA to 200mA in total as there are 4 parallel transistors.
I'm having trouble seeing the detail on the schematic regarding the potentiometers. One will certainly be to set the output transistor bias current an is very important. If you could scan a smaller area of the schematic and post it or email it to me I'll try and explain what each pot is for. For the bias current you probably want about 30mA to 50mA per transistor - or 120mA to 200mA in total as there are 4 parallel transistors.
I'm no expert on bridging amps but I reason that small differences in supply rail volatage should not be important. I imagine it is more likely that some instability could occur when you have two feedback systems interacting with one another and this may be made worse by the smaller load that each amp sees. You need to be sure that each individual amp is designed to drive half it's normal load without difficulty.
There appear to be two main challenges when bridging that need to be addressed carefully:
1) Overheating/over current (as already discussed). When you bridge two amps you quadruple the power dissipations and circuits not carefully designed to be bridged won't survive this for long.
2) Ground paths. It is important to try to draw the complete path of the current flow of the speaker and see where it goes. In bridging you can end up with two separate star grounds and if there is high current flow between them, without low enough resistance connection, you can end up with instability and feedback from output to input - in a benign form this results in some buzzing in the speaker. So I imagine you need to ensuse both amps are very close to one another and have very thick cable between the 0V speaker terminals or a direct connection between ground points.
Amp runs on ±80V nominal, so bridged it can swing ±160V into the load on peaks.
Lets say we are driving ±75V peaks into two 8 ohm speakers, about 3 ohms below 100hz. The power supply sags to ±150V maximum swing. Peak current to the speakers is 25A, with 75V across the outputs.
Peak output dissipation is then 25A X 75V = 1875W, average power is 1325W. Power is handled over each half cycle by 8 outputs, 1325 divided by 8 is 165W per device.
The thermal resistance of the MJ15024 is 0.7*C/W
If it was directly mounted to the heatsink with thermal compound only that would add 0.1*C/W
If the heatsink was water cooled and held at 25*C the junction temperature would be 0.8*C/W X 165W = 132*C + 25*C (ambient), or 157*C average.
Over time and temperature a real insulator washer is about 0.35*C/W (assuming the grease was applied by a skilled operator and it was mounted and torqued properly to a clean, flat sink.
The extra 0.25*C/W is another 41.25*C to the junction temperature, or 198.25*C
Is your ambient really 25*C? Most racks I measure are closer to 55*C when running for a long time.
228.25*C junction temperature.
Do you really have a water cooled heatsink?
I think we just screwed the pooch. A real, real, big forced air sink is going to be at least 0.05*C/W, times the 1325W the sink has to get rid of is 66.25*C above ambient.
294.5*C junction temperature.
Bass in a bi-amp system below 100hz can have a 50% duty cycle. cRAP music for instance. This will reduce the junction temperature to about 208*C.
If we assume a 25* duty cycle then our junction temperature looks like 147.25*C, not too bad.
But remember, that was with an unrealistic 0.05*C/W sink. Try and find one and see how much it costs. 0.1*C/W is more 'real'.
That brings us up to 213.5*C junction temperature again.
And you thought I was kidding about the 39 hour life at 4 ohms bridged?
Driving only one woofer in bridge will bring the life up to about 625 hours, a 16X improvement.
Driving one woofer per channel in stereo will bring the life up to about 10,000 hours (no kidding!).
Newer amplifiers with class G and H output stages run cooler.
Lets say we are driving ±75V peaks into two 8 ohm speakers, about 3 ohms below 100hz. The power supply sags to ±150V maximum swing. Peak current to the speakers is 25A, with 75V across the outputs.
Peak output dissipation is then 25A X 75V = 1875W, average power is 1325W. Power is handled over each half cycle by 8 outputs, 1325 divided by 8 is 165W per device.
The thermal resistance of the MJ15024 is 0.7*C/W
If it was directly mounted to the heatsink with thermal compound only that would add 0.1*C/W
If the heatsink was water cooled and held at 25*C the junction temperature would be 0.8*C/W X 165W = 132*C + 25*C (ambient), or 157*C average.
Over time and temperature a real insulator washer is about 0.35*C/W (assuming the grease was applied by a skilled operator and it was mounted and torqued properly to a clean, flat sink.
The extra 0.25*C/W is another 41.25*C to the junction temperature, or 198.25*C
Is your ambient really 25*C? Most racks I measure are closer to 55*C when running for a long time.
228.25*C junction temperature.
Do you really have a water cooled heatsink?
I think we just screwed the pooch. A real, real, big forced air sink is going to be at least 0.05*C/W, times the 1325W the sink has to get rid of is 66.25*C above ambient.
294.5*C junction temperature.
Bass in a bi-amp system below 100hz can have a 50% duty cycle. cRAP music for instance. This will reduce the junction temperature to about 208*C.
If we assume a 25* duty cycle then our junction temperature looks like 147.25*C, not too bad.
But remember, that was with an unrealistic 0.05*C/W sink. Try and find one and see how much it costs. 0.1*C/W is more 'real'.
That brings us up to 213.5*C junction temperature again.
And you thought I was kidding about the 39 hour life at 4 ohms bridged?
Driving only one woofer in bridge will bring the life up to about 625 hours, a 16X improvement.
Driving one woofer per channel in stereo will bring the life up to about 10,000 hours (no kidding!).
Newer amplifiers with class G and H output stages run cooler.
Traderbam,
It's difficult to get a decent scan or the drawing to fin in the 100k limit here so if I had your email address iI could mail you the whole manual (800K). Anyway, I have tried to clean up a clip of the shematics witch I attach here.
I will allso take the opertunity to tank you all for the help here and DJK, I think I realized now what killed my amp..
/Peter
It's difficult to get a decent scan or the drawing to fin in the 100k limit here so if I had your email address iI could mail you the whole manual (800K). Anyway, I have tried to clean up a clip of the shematics witch I attach here.
I will allso take the opertunity to tank you all for the help here and DJK, I think I realized now what killed my amp..
/Peter
Attachments
VR3 & VR4 look like the bias current adjustments. These are the most important things with regard to you avoiding initial melt-down.
If you haven't already power it up and adjusted the bias current, here's what I'd do:
1) put a 10-ohm, 10W+ resistor in series with the +'ve supply to the upper output stage, or the whole amp if easier.
2) Adjust VR3 and VR4 to be as big a value as possible.
3) Connect a voltmeter across the 10-ohm resistor
4) Power on
5) adjust VR3 until you get the specified current (see the manual). I image it will be about 200mA - so you'd see 2 volts across the 10-ohm resistor.
6) repeat for lower stage.
7) remove resistor.
This is a relatively safe technique because if there is a fault or anything you'll lose just the 10-ohm resistor and save yourself another major soldering job. 🙂
My email: traderbam@yahoo.co.uk
If you haven't already power it up and adjusted the bias current, here's what I'd do:
1) put a 10-ohm, 10W+ resistor in series with the +'ve supply to the upper output stage, or the whole amp if easier.
2) Adjust VR3 and VR4 to be as big a value as possible.
3) Connect a voltmeter across the 10-ohm resistor
4) Power on
5) adjust VR3 until you get the specified current (see the manual). I image it will be about 200mA - so you'd see 2 volts across the 10-ohm resistor.
6) repeat for lower stage.
7) remove resistor.
This is a relatively safe technique because if there is a fault or anything you'll lose just the 10-ohm resistor and save yourself another major soldering job. 🙂
My email: traderbam@yahoo.co.uk
Traderbam,
Am I supposed to have any load connected to the speaker terminals when I'm doing this ?
I'll guess I have to go for your 200mA because the manual says noathing about it.
/Peter
Am I supposed to have any load connected to the speaker terminals when I'm doing this ?
I'll guess I have to go for your 200mA because the manual says noathing about it.
/Peter
1500 Amp
This is a really old amplifier that the company bought on an OEM basis from a British company. The design was not very good and if I was working here at the time I would have vetoed the purchase. This amplifier has no short circuit protection to speak of, no SOA protection and it uses a driver transformer (yuk). I was building more advanced stiff in the 70's and these were built in the mid 80s.
In bridge mode, this amplifier should not be operated into a 4 ohm load. It will blow up. It can safely operate 4 ohm loads one ach channel in normal oeration but in bridge mode, the effective impedance seen by each channel is halved. Each channel sees a 2 ohm load which is too small a load impedance for an amplifier this old.
However, other than the driver transformer (which we cannot get) the amplifier can be repaired. Do replace all the transistors in that channel and all the open emitter resistors. Do note that a blown emitter resistor is the result of failure elsewhere, not the cause of one. Also note that this amp has had cases of bad bias resistors. Check that the other power resistors in that channel are the correct resistance.
If you need a schematic, please email me at dmf@renkus-heinz.com.
Ourselves, we made better amplifiers not long after that one and also relaced the engineer who said that this amplifier was OK to buy.
This is a really old amplifier that the company bought on an OEM basis from a British company. The design was not very good and if I was working here at the time I would have vetoed the purchase. This amplifier has no short circuit protection to speak of, no SOA protection and it uses a driver transformer (yuk). I was building more advanced stiff in the 70's and these were built in the mid 80s.
In bridge mode, this amplifier should not be operated into a 4 ohm load. It will blow up. It can safely operate 4 ohm loads one ach channel in normal oeration but in bridge mode, the effective impedance seen by each channel is halved. Each channel sees a 2 ohm load which is too small a load impedance for an amplifier this old.
However, other than the driver transformer (which we cannot get) the amplifier can be repaired. Do replace all the transistors in that channel and all the open emitter resistors. Do note that a blown emitter resistor is the result of failure elsewhere, not the cause of one. Also note that this amp has had cases of bad bias resistors. Check that the other power resistors in that channel are the correct resistance.
If you need a schematic, please email me at dmf@renkus-heinz.com.
Ourselves, we made better amplifiers not long after that one and also relaced the engineer who said that this amplifier was OK to buy.
Well,when I powerd the amp up after replacing all the transistors, (trough a variable power source, didn't dare to go right on..), I found that one channel of the amp connected directly to the -ve rail (-ve rail voltage at speaker terminal ), but the transistors wasn't shorted.
After some investigations I found the bias resistors (Traderbam, If you look at the drawing it's the 5R6 2.5W R107 and the 100R Pot VR4 that is broken)was burnt out .
My guess is that the resistor R107 broke because of age or whatever reason, witch let the pot VR4 take all the load witch ofcourse led to the destruction of the 0,25W pot.
Now there was nothing to stop TR13 - TR16 from going fully "ON", and since the amp was in bridge mode, the disaster was unavoidable.
This is my theory of the event, What do you say, am I totally wrong here or ?
/Peter
After some investigations I found the bias resistors (Traderbam, If you look at the drawing it's the 5R6 2.5W R107 and the 100R Pot VR4 that is broken)was burnt out .
My guess is that the resistor R107 broke because of age or whatever reason, witch let the pot VR4 take all the load witch ofcourse led to the destruction of the 0,25W pot.
Now there was nothing to stop TR13 - TR16 from going fully "ON", and since the amp was in bridge mode, the disaster was unavoidable.
This is my theory of the event, What do you say, am I totally wrong here or ?
/Peter
Dmfraser,
I posted my latest reply before I read yours.
I saw that you mentioned the potentially bad bias resistor, and that was exactly what happend in my case.
In the last year I have replaced the 15000uf caps, all power resistors (exept the bias resistors....), and all the transistors. When I get this amp to work again I will probably take it down to my local dealer and trade it for a new one. Funny is that when I was talking to the dealer a couple of years ago, he offered me lot more than I expected for it. He said "Its a Renkus-Heinz, Thats the rolls royce of amplifiers so Ill pay you good money for it". Do you think I should I tell him that it's not an R-H but an old crappy Hill amp before or after I made the deal with him ? 😉
/Peter
I posted my latest reply before I read yours.
I saw that you mentioned the potentially bad bias resistor, and that was exactly what happend in my case.
In the last year I have replaced the 15000uf caps, all power resistors (exept the bias resistors....), and all the transistors. When I get this amp to work again I will probably take it down to my local dealer and trade it for a new one. Funny is that when I was talking to the dealer a couple of years ago, he offered me lot more than I expected for it. He said "Its a Renkus-Heinz, Thats the rolls royce of amplifiers so Ill pay you good money for it". Do you think I should I tell him that it's not an R-H but an old crappy Hill amp before or after I made the deal with him ? 😉
/Peter
Renkus-Heinz Amps
If you tell him its a Hill amp or not is up to you. If he's a pro dealer in Europe he should recognize it. The P1500 was only sold for a couple years in the 1980s. Since then, Renkus-Heinz has manufactured their own amplifiers as well as sourced some from Crest in the USA and FBT in Italy. Currently we only make amplifiers for our powered loudspeakers. For these we use Class-D modules from Bang & Olufsen's ICE Power division.
Renkus-Heinz no longer manufactures or sells any rack amplifiers. They were too good and with the flood of cheap Chinese made amplifiers we could no longer make any money selling a top of he line, high quality, price no object pro amplifier. However, what we sold before shutting down that part of the business, the P3500, P2950, etc. were about the finest linear pro power amplifiers ever made. Some of the models can still be purchased in Europe under the FBT brand name.
If you tell him its a Hill amp or not is up to you. If he's a pro dealer in Europe he should recognize it. The P1500 was only sold for a couple years in the 1980s. Since then, Renkus-Heinz has manufactured their own amplifiers as well as sourced some from Crest in the USA and FBT in Italy. Currently we only make amplifiers for our powered loudspeakers. For these we use Class-D modules from Bang & Olufsen's ICE Power division.
Renkus-Heinz no longer manufactures or sells any rack amplifiers. They were too good and with the flood of cheap Chinese made amplifiers we could no longer make any money selling a top of he line, high quality, price no object pro amplifier. However, what we sold before shutting down that part of the business, the P3500, P2950, etc. were about the finest linear pro power amplifiers ever made. Some of the models can still be purchased in Europe under the FBT brand name.
It certainly does.
IMO, for amps in general,
Barely acceptable dc offset is +/-100mV.
Ok offset is +/- 50mV.
Good offset is +/-5mV.
Are you getting the high offset on both channels or just the -'ve one? If only the -'ve one then it should be easy to find what is different between two: measure all the resistors vlaues with the power off and then all the voltages with the power on.
What bias current have you set?
IMO, for amps in general,
Barely acceptable dc offset is +/-100mV.
Ok offset is +/- 50mV.
Good offset is +/-5mV.
Are you getting the high offset on both channels or just the -'ve one? If only the -'ve one then it should be easy to find what is different between two: measure all the resistors vlaues with the power off and then all the voltages with the power on.
What bias current have you set?
Perhaps, but it may also have been a blown transistor whose base forced 80V across both R107 and VR4 - which R107 would not have liked at all!
traderbam,
I'w found the cause for the bad dc offset, a bad resistor I used for the current limiting during testing. However, the current drawn over the 10ohm resistor in series with +ve rail ins't affected
by adjusting vr3 and vr4 ! Its stable at 200mA (+-10mA) regardless of the setting of vr3 and vr4. But the DC offset at the speaker terminals is possible to adjust down to a few mV by adjusting vr3 and vr4 respectivly. Do you think that setting the dc offset is the only way to adjust this amp ?
/peter
I'w found the cause for the bad dc offset, a bad resistor I used for the current limiting during testing. However, the current drawn over the 10ohm resistor in series with +ve rail ins't affected
by adjusting vr3 and vr4 ! Its stable at 200mA (+-10mA) regardless of the setting of vr3 and vr4. But the DC offset at the speaker terminals is possible to adjust down to a few mV by adjusting vr3 and vr4 respectivly. Do you think that setting the dc offset is the only way to adjust this amp ?
/peter
Temperature stabillity seems to be a problem in this amp, If I let the amp to heat up for one hour and than adjust the dc bias to +- 20mV, when I then cold start it, the dc bias is about -200mV or more. The resistors R16, R18, R88 and R89 (820R 25W) gets really hot and are located on the heatsink near the transistors. The fans dosen't start until the amp gets quite hot so I wondered if it might be a good idea to hotwire the fans so they run all the time (fan noise dosen't really matters in this case).
/Peter
/Peter
modified the fan driver circuit, now the fan regulation is from 30 to 100% not 0 to 100% as it was originally . Intresting to see if that will make any differance.
/Peter
/Peter
There is no active offset control in the design. So the offset will vary as the transistors change temperature, because the transistors are not identical to one another. A 200mV offset isn't dangerous or anything - it won't damage your speakers.
- How quickly does it reduce after power on?
- What is the initial offset when a speaker is connected?
- is the offset the same on both channels?
- How quickly does it reduce after power on?
- What is the initial offset when a speaker is connected?
- is the offset the same on both channels?
Traderbam, When cold, both channels are about +270mV (left 260mV, Right 280mV). After 30 minutes, left channel is about -15mV and right -20mV. After one hour of operation both channels is about -30mV.
This is the best I can get it.
I havn't tested it with load yet but tomorrow I will do so.
/Peter
This is the best I can get it.
I havn't tested it with load yet but tomorrow I will do so.
/Peter
It's good that it is consistent between the two channels because it suggests the dc drift is in the design itself rather than your soldering 🙂 It is also good that the drift decreases as the amp warms up and stabilizes at 30mV. I also think this is the best you'll do and it is ok.
What I would do next is get the amp really toasty and check both the offset and the bias current. It is difficult to run the thing at power and have a current measuring resistor in the psu, so you may have to measure the voltage across a few emitter resistors and average it to get an approximate current value. If the bias current is 200mA +/- 100mA or better from cold to hot then that's fine. If the dc offset stays below +/-100mV from hot to cold than that's ok. Take a hairdryer to the heatsink to speed up this process.
You may have noticed that since the polarity of dc offset tracks well between the two channels it is pretty small in bridged mode. But as per previous comments the amp shouldn't be run at power with less than 8-ohms in bridge mode.
What I would do next is get the amp really toasty and check both the offset and the bias current. It is difficult to run the thing at power and have a current measuring resistor in the psu, so you may have to measure the voltage across a few emitter resistors and average it to get an approximate current value. If the bias current is 200mA +/- 100mA or better from cold to hot then that's fine. If the dc offset stays below +/-100mV from hot to cold than that's ok. Take a hairdryer to the heatsink to speed up this process.
You may have noticed that since the polarity of dc offset tracks well between the two channels it is pretty small in bridged mode. But as per previous comments the amp shouldn't be run at power with less than 8-ohms in bridge mode.
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