Andrew, could you show us how your design look like? If you have done it right, this should not happen you know.
Which value has your fuses and where have you put them?
If you happend to have a LM3875 or 3886 it is VERY important that you never break the current path from the inputs of the IC. MUST NOT DO THAT.
Which value has your fuses and where have you put them?
If you happend to have a LM3875 or 3886 it is VERY important that you never break the current path from the inputs of the IC. MUST NOT DO THAT.
Hi Anatech,
just got back to my thread.
It is the act of plugging in an input cable, while powered up, that blows the fuse.
The internal mods are ONLY carried out while completely disconnectted from the mains and system and time to charge down to zero.
just got back to my thread.
It is the act of plugging in an input cable, while powered up, that blows the fuse.
The internal mods are ONLY carried out while completely disconnectted from the mains and system and time to charge down to zero.
Do you have any mains filter? Any potential between the preamp amp and the power amp? If you have mains filter is the amp connected to protective ground?
Again, the fuse value and where?
Again, the fuse value and where?
Hi all,
thanks for all the contributions that are starting to flow in. Can we talk about this subject at a sensible UK time? 😉
All that chatter and I don't get a chance to join in 🙁
The amps are a commercial design and assembly.
I have drawn a schematic from the PCB and it seems to be very well designed. It is completely stable with no input connected. Sound impeccable in my system. About 200W into 8ohms but NOT suitable for lower load values.
The design is, I guess, over 20years old but the manufacturer was not willing to acknowledge any design info or even the existence of this older amp. The model is Sugden P128. About 200W into 8ohms. NOT suiatable for lower load values.
The structure is
DC block - 5534 opamp gain *2 with 22pF comp.- DC block + RF filter - LTP and CCS(fet) with resistor collector load. - VAS with resistor bias in series to CCS (2BJT) - complementary EF driver pair - 2 complementary SF output pairs (2sk225/J81) now replaced with 4pairs 2sk1058/J162.
PSU is rect/cap for outputs +-73Vdc and extra low voltage tapping added on top of main PSU giving input and driver stage +-83Vdc . Resistor string to reduce to +-21Vdc for opamp.
All these rail voltages drop when the output is biased up to 300mA giving +-69Vdc, +-77Vdc, +-18Vdc. The VAS and VAS CCS run hot to touch due to a dissipation of about half maximum. Output sinks run at about 40degC, Tc about 43degC.
thanks for all the contributions that are starting to flow in. Can we talk about this subject at a sensible UK time? 😉
All that chatter and I don't get a chance to join in 🙁
The amps are a commercial design and assembly.
I have drawn a schematic from the PCB and it seems to be very well designed. It is completely stable with no input connected. Sound impeccable in my system. About 200W into 8ohms but NOT suitable for lower load values.
The design is, I guess, over 20years old but the manufacturer was not willing to acknowledge any design info or even the existence of this older amp. The model is Sugden P128. About 200W into 8ohms. NOT suiatable for lower load values.
The structure is
DC block - 5534 opamp gain *2 with 22pF comp.- DC block + RF filter - LTP and CCS(fet) with resistor collector load. - VAS with resistor bias in series to CCS (2BJT) - complementary EF driver pair - 2 complementary SF output pairs (2sk225/J81) now replaced with 4pairs 2sk1058/J162.
PSU is rect/cap for outputs +-73Vdc and extra low voltage tapping added on top of main PSU giving input and driver stage +-83Vdc . Resistor string to reduce to +-21Vdc for opamp.
All these rail voltages drop when the output is biased up to 300mA giving +-69Vdc, +-77Vdc, +-18Vdc. The VAS and VAS CCS run hot to touch due to a dissipation of about half maximum. Output sinks run at about 40degC, Tc about 43degC.
Are you really sure the new output transistors work good and do not oscillate?
How does the input look like? DC blocking caps, any other filters? Pulldown resistor for the input cap?
How does the input look like? DC blocking caps, any other filters? Pulldown resistor for the input cap?
Hi Peranders,
as before the input is - DC block - resistor to ground - resistor to opamp input pin. But no resistor before DC block to ground. I plan to add a 2M2 from RCA input pin to ground to pull down any leakage current passing the DC block (from either the preamp or the power amp).
Between the first stage opamp gain (*2) and the main power amp is another set of - DC block - with resistor to ground - resistor in series - cap to ground forming RF filter - resistor to LTP +ve input.
The 2pair output stage (original) also blew the fuse on inserting a cable into the input RCA. That still happens to the 4 pair version and still happens after adding the input RF filter referred to earlier. But it reliably runs for days within the system with a cable connected or on the test bed with no cable connected.
I have not yet tested the amp into reactive loads nor FULL power into 8r. I would like to solve this offset problem first.
Does anyone really need the schematic. I could take a pic of my hand drawn schematic and post it (since Sugden don't even accept it as their handiwork). Had they been more co-operative I would need their permission before posting.
as before the input is - DC block - resistor to ground - resistor to opamp input pin. But no resistor before DC block to ground. I plan to add a 2M2 from RCA input pin to ground to pull down any leakage current passing the DC block (from either the preamp or the power amp).
Between the first stage opamp gain (*2) and the main power amp is another set of - DC block - with resistor to ground - resistor in series - cap to ground forming RF filter - resistor to LTP +ve input.
The 2pair output stage (original) also blew the fuse on inserting a cable into the input RCA. That still happens to the 4 pair version and still happens after adding the input RF filter referred to earlier. But it reliably runs for days within the system with a cable connected or on the test bed with no cable connected.
I have not yet tested the amp into reactive loads nor FULL power into 8r. I would like to solve this offset problem first.
Does anyone really need the schematic. I could take a pic of my hand drawn schematic and post it (since Sugden don't even accept it as their handiwork). Had they been more co-operative I would need their permission before posting.
Hi Andrew
Could be the NE5534 going high (latch-up). Try by-passing it completely.
Regards
Could be the NE5534 going high (latch-up). Try by-passing it completely.
Perhaps small transformer. This will limit current to the output so even at 4 ohms, the outputs are safe.
Regards
Hi Mcp,
I would like,as an experiment, to include the opamp inside the overall feedback loop using the topology described by Walt Jung. This is a fairly complicated PCB alteration that may follow a number of other changes that are much easier to implement.
Your idea to exclude it completely would be fairly easy to do although the input cable route will be a bit untidy. For instance the designer has stripped back about 25mm of coax and arranged the core and screen as a parallel pair and overlaid them exactly above the input track and ground track of the DC block high pass filter to form what looks like a quad 4 cable arrangement. Initially this odd layout (insulated core with no screen) seemed very strange and only prolonged study of my schematic and comparing this to the board layout did the quad 4 layout become apparent. I cannot believe that this layout is an accident or laziness on the part of the PCB assembler. It appears to be by design, in my opinion very clever design to cancel interference pickup. To disconnect and move the core (easy because it's long enough) would defeat the quad 4 layout. Tell me again to still exclude the opamp and I may follow your next recommendation.
I cannot recall seeing a previous reference to 5534 suffering from latch up. Can you confirm this behaviour?
Your suggestion to consider Vrail droop as a justification for low inpedance loads is completely unfounded. My use of Bensen's spreadsheet which includes a correction for transformer regulation shows that the amp on 4pairs cannot reliably survive 4ohm reactive loading even fairly short term. The 8ohm Vrail=63.2Vdc at 60deg phase angle Tc=45degC. The 4ohm Vrail has fallen further to 61.2Vdc at Tc=45degC but is just outside the 100mS SOAR at the same 60degree phase angle. Add in the time for the PSU to fall from 69Vdc to 61.2Vdc leads to my opinion that this is not acceptable for long term reliability. I believe that this amp was never designed for lower impedance loads (in original 2pair it is even worse). The two transformers (about 175VA each) supplying the dual discrete bridge further support my contention. It would be nice to hear the designer's thoughts or even a see a specification sheet or sales literature but Sugden will not play ball.
I would like,as an experiment, to include the opamp inside the overall feedback loop using the topology described by Walt Jung. This is a fairly complicated PCB alteration that may follow a number of other changes that are much easier to implement.
Your idea to exclude it completely would be fairly easy to do although the input cable route will be a bit untidy. For instance the designer has stripped back about 25mm of coax and arranged the core and screen as a parallel pair and overlaid them exactly above the input track and ground track of the DC block high pass filter to form what looks like a quad 4 cable arrangement. Initially this odd layout (insulated core with no screen) seemed very strange and only prolonged study of my schematic and comparing this to the board layout did the quad 4 layout become apparent. I cannot believe that this layout is an accident or laziness on the part of the PCB assembler. It appears to be by design, in my opinion very clever design to cancel interference pickup. To disconnect and move the core (easy because it's long enough) would defeat the quad 4 layout. Tell me again to still exclude the opamp and I may follow your next recommendation.
I cannot recall seeing a previous reference to 5534 suffering from latch up. Can you confirm this behaviour?
Your suggestion to consider Vrail droop as a justification for low inpedance loads is completely unfounded. My use of Bensen's spreadsheet which includes a correction for transformer regulation shows that the amp on 4pairs cannot reliably survive 4ohm reactive loading even fairly short term. The 8ohm Vrail=63.2Vdc at 60deg phase angle Tc=45degC. The 4ohm Vrail has fallen further to 61.2Vdc at Tc=45degC but is just outside the 100mS SOAR at the same 60degree phase angle. Add in the time for the PSU to fall from 69Vdc to 61.2Vdc leads to my opinion that this is not acceptable for long term reliability. I believe that this amp was never designed for lower impedance loads (in original 2pair it is even worse). The two transformers (about 175VA each) supplying the dual discrete bridge further support my contention. It would be nice to hear the designer's thoughts or even a see a specification sheet or sales literature but Sugden will not play ball.
Hi Clem,
since you have asked, I will try to organise it but be a little patient.
The last time I emailed a circuit diagram the file was 300kb but uncompressed so I hope I can get below our limit and still be legible.
It's nothing other than a three stage LIN type topology with careful placement of compensation.
since you have asked, I will try to organise it but be a little patient.
The last time I emailed a circuit diagram the file was 300kb but uncompressed so I hope I can get below our limit and still be legible.
It's nothing other than a three stage LIN type topology with careful placement of compensation.
Hi AndrewT,
looking at it now... have you ever tried pushing a square wave into the inputs? Wondering if there's a possibility the outputs are failing to turn off quickly enough at HF. Those are pretty small driver transistors...
Cheers!
looking at it now... have you ever tried pushing a square wave into the inputs? Wondering if there's a possibility the outputs are failing to turn off quickly enough at HF. Those are pretty small driver transistors...
Cheers!
Hi Clem,
no square waves, yet. I wanted to solve the fuse blowing problem first.
Your absolutely right - TINY drivers. To92=600mW
Maybe that's why the RF filter F3=50kHz.
no square waves, yet. I wanted to solve the fuse blowing problem first.
Your absolutely right - TINY drivers. To92=600mW
Maybe that's why the RF filter F3=50kHz.
Hi Andrew
Looks like the problem is at the opamp side. Probably quite high ground hum emitting from opamp.
To avoid blowing any output mosfets, temporarily remove input capacitor C5 (600nF). Plug in the interconnect with the ground (shield) disconnected. This will induce the hum. Check the opamp output with a scope.
You may wish to consider doing away with the opamp altogether. If you want to retain the 2X gain of the opamp, this can easily be achieved by slight adjustments on the discreet side. Perhaps it's meant to be an input buffer. If so, the impedance is not very high. Nonetheless, an opamp right at the front will influence the entire circuit, sonic wise.
It's quite obvious that the original design is very conservative. The input filter is set for 12Hz-56KHz(-3dB). In use, the low frq will start to roll of at about 120Hz and the highs at 5.6KHz.
Mosfet gate stopper resistors are also very heavy (1K+470 ohms). This will also severely limit the high frq response of the mosfets.
Regards
Looks like the problem is at the opamp side. Probably quite high ground hum emitting from opamp.
To avoid blowing any output mosfets, temporarily remove input capacitor C5 (600nF). Plug in the interconnect with the ground (shield) disconnected. This will induce the hum. Check the opamp output with a scope.
You may wish to consider doing away with the opamp altogether. If you want to retain the 2X gain of the opamp, this can easily be achieved by slight adjustments on the discreet side. Perhaps it's meant to be an input buffer. If so, the impedance is not very high. Nonetheless, an opamp right at the front will influence the entire circuit, sonic wise.
It's quite obvious that the original design is very conservative. The input filter is set for 12Hz-56KHz(-3dB). In use, the low frq will start to roll of at about 120Hz and the highs at 5.6KHz.
Mosfet gate stopper resistors are also very heavy (1K+470 ohms). This will also severely limit the high frq response of the mosfets.
Regards
I cannot download the schematic through our security system at work, but I can speculate why the output sits at the +ve rail when the -ve rail fuse blows. My theory is that the VAS CCS is fed/referenced from the 0V rail, so that when the -ve rail loses power the VAS transistor no longer is able to sink current, but the CCS is still trying to source current. This forces the upper output device hard on.
The way to avoid this is to reference the CCS to the -ve rail so popped fuse = no current flow, but this halves PSRR.
The way to avoid this is to reference the CCS to the -ve rail so popped fuse = no current flow, but this halves PSRR.
Sugden Told me that the amps are 140watts into 8r and 240watts into4r so i cant see a problem with running 4r speakers?
Regards
Danny
Regards
Danny
a Question of class
And I'm assuming that these are similar to the A28 II and P28 in being high biased class AB (ie a few watts of class A - any idea of how many?)
How do these differ from the A28 II. Yes - i know that the P128s are mono-block power amps and the A28 II is an integrated pre/power - I'm more interested in the differences in the specs of the power sections and why the A128s were seen as better by Sugden at the time. Was it just a question of more (W) is better?
And the reason I ask - I've owned a A28II from new and loved it - serviced and working fine. But I'm now looking at a pair of the the P128s.
And I'm assuming that these are similar to the A28 II and P28 in being high biased class AB (ie a few watts of class A - any idea of how many?)
How do these differ from the A28 II. Yes - i know that the P128s are mono-block power amps and the A28 II is an integrated pre/power - I'm more interested in the differences in the specs of the power sections and why the A128s were seen as better by Sugden at the time. Was it just a question of more (W) is better?
And the reason I ask - I've owned a A28II from new and loved it - serviced and working fine. But I'm now looking at a pair of the the P128s.
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The P128 is very different from the A28
The P128 is dual Lateral mosFET ClassAB amplifier, biased to 75mA per device.
Maximum ClassA current is ~300mApk, or about 0.36W into 8r0
Check your supply voltages, particularly on the main smoothing caps with the secondary fuses removed/blown and on the 5534 opamp.
I fitted Zeners to protect the opamp from overvoltage.
The P128 is dual Lateral mosFET ClassAB amplifier, biased to 75mA per device.
Maximum ClassA current is ~300mApk, or about 0.36W into 8r0
Check your supply voltages, particularly on the main smoothing caps with the secondary fuses removed/blown and on the 5534 opamp.
I fitted Zeners to protect the opamp from overvoltage.
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