I purchased a Mission 777 power amplifier on Ebay last fall. It's a great sounding amp that matches well with my Mission 776 preamp. It is a completely symmetrical dual mono design direct coupled with no input or output capacitors. It uses 2 pairs of Hitachi mosfets per channel. There is no output Zobel or output inductor. There are no potentiometers. Individual resistors are chosen in the input stage to balance the amp. I recently decided to check the DC offset at the outputs and was quite surprised what I measured. With no inputs connected the DC offset is 0.359 VDC on the left and -0.359 on the right. With the Mission 776 preamp connected, the offset rises to 1.296 VDC on the left and -1.296 on the right. With the amps inputs shorted the offset rises to 5.23 VDC on the left and -5.23 on the right. I'm not used to seeing offsets this high for DC amps. All my other amps measure very low. My Cyrus 2 measures 6mV and -12mV and my Outlaw measure 20mV. I wonder if I should make some adjustments to the input stage to see if I can reduce this offset?
Hi,
Does it play O.K. no distortion etc. The offset's you are quoting are horrendous, it's not out of adjustment, there a serious fault, whether a design fault or component fault you can't say.
Have I read this right, 5 Volts across the speaker with the inputs shorted. You are putting your speakers at risk, the heating effect of half an amp plus, will overheat the voice coils.
You have come come up with a circuit I'm afraid, but it's weird both channels affected, perhaps there a common reference voltage somewhere thats misbehaving.
Regards Karl
Does it play O.K. no distortion etc. The offset's you are quoting are horrendous, it's not out of adjustment, there a serious fault, whether a design fault or component fault you can't say.
Have I read this right, 5 Volts across the speaker with the inputs shorted. You are putting your speakers at risk, the heating effect of half an amp plus, will overheat the voice coils.
You have come come up with a circuit I'm afraid, but it's weird both channels affected, perhaps there a common reference voltage somewhere thats misbehaving.
Regards Karl
I recently purchased a copy of the schematic on Ebay from another owner of this amp in Europe. The schematic is not a factory original. It was apparently drawn by a service technician and has an error in the feedback circuit which I have yet to clarify by openning up my own amp and trying to trace out the feedback loop myself. From the schematic that I have there are no capacitors in the signal path, even in the feedback loop. The design is completely mirror symmetrical but rather unconventional. Describing only the upper half, the input stage is an pnp emitter follower driven by half the negative rail voltage. This cascades into a npn common emitter stage which is biased in reference to half the positive rail through a resistor value which is apparently chosen at the factory. This stage then cascades into a pnp common emitter stage cascode which is loaded by a current mirror. The output of this cascode drives the voltage gain/driver stage which is three parallel npn transistors(2SD699) tied to the negative rail in common emitter configuration. The combined collector output of these parallel transistors then feeds a pair of p-type Hitachi 2SJ50 mosfets in parallel. The feedback loop is voltage divider which comes from the output back to the bases of the input transistors. It would seem to me that the DC offset is adjusted by changing the value of the resistors that bias the first common emitter stage. I've tried to put this circuit up on LTSpice but I can't get it to work because of the error in the feedback loop which I haven't had time to clarify. I think that I understand the circuit, unfortunately I don't have that much experience repairing or servicing amplifiers.
people actually do that As far as I can tell it's all original. The amp looked mint inside. I didn't see any sign of modding but then I never took that close of a look. It looked identical to reference pictures that I have. The amp sounds great. Wide open presentation, dynamic, lots of detail. I drew the schematic up on LTSpice but there is a missing node in the feedback loop. I assume there must be a capacitor to ground missing otherwise there wouldn't be unity gain at DC. With the amp connected and running the DC offset sits steady at 1.296 VDC on the left and -1.296 on the right. These aren't particularly good figures so I'm worried about my speakers.
Ok, I broke down and disassembled my amp this afternoon. I can't stand the thought of driving that much DC into my speakers. I pulled out the main board on one of the channels and was able to trace out the feedback circuit and check all the values on my schematic with the actual board. There were a couple differences in resistor values but relatively small. A 9.31K resistor comes off the source of the two N-power mosfets to the feedback loop. The sources of the N-power mosfet and the P-power mosfets don't meet until they come to the safety fuse. The output of the fuse goes directly to the positive speaker terminal and a second feedback loop with a 68.1 ohm resistor joins the 9.31K resistor. I guess this is the primary feedback loop with the second loop only coming into play if the fuse blows. I ran the circuit on LTSpice and it produced about 4V of positive DC offset. I threw an electrolytic capacitor onto the voltage divider resistor to ground and this corrected the DC offset. Unfortunately I don't know what this change will make to the stability of the amp. Also I don't know how accurate this model is to the real worldcircuit. I've attached a revised schematic.
I've just had a look on the mainboard of the other channel. It seems that they vary the combination of R5,R6 and R9,R10 to set the DC offset because these resistor values are different once again. By playing with the values on LTSpice I can reduce the offset to zero but that assumes ideal conditions which may not be the same as real world. I can adjust either of the two resistors to zero the offset. I'm not sure which one is the better resistor to change to a pot.
Thanks Mooly. The attachment is a Spice simulation. I transferred the schematic to the simulation to find the best way to tune out the DC offset. This was supposed to have been done at the factory by part matching in the final testing stages. Maybe it was OK when it left the factory. The amp is almost 20 years old so things certainly could have drifted. The DC offset can be adjusted by either changing the resistor value of the emitter resistor(R6) of the input transistor or by changing the bias resistor (R5) to the second stage or a combination of both. I can't say I understand enough about design to say which is the better way.
It's not how would do things if it's so critical, having to select on test like this is, well, we wont even go there. I am sure they had their reasons.
Without a factory manual I have to go off what you say, and I suspect there's more to it than this.
Nobody else any thoughts ? What I think will happen is this, it's not just setting for zero offset, which you can achieve by altering one component, but getting that offset to stay at zero through the full operational temperature range. Its going to drift I suspect.
It might be worth replacing R5/10 with multi turn pots set initially to values already there and then have a play. Tweak just a little, try to get a feel for what happens. Anybody ? You could perhaps try and match the input currents (voltage across R7/8 ) Is altering these going to alter the current flowing in the outputs as well, something to watch I think.
Without a factory manual I have to go off what you say, and I suspect there's more to it than this.
Nobody else any thoughts ? What I think will happen is this, it's not just setting for zero offset, which you can achieve by altering one component, but getting that offset to stay at zero through the full operational temperature range. Its going to drift I suspect.
It might be worth replacing R5/10 with multi turn pots set initially to values already there and then have a play. Tweak just a little, try to get a feel for what happens. Anybody ? You could perhaps try and match the input currents (voltage across R7/8 ) Is altering these going to alter the current flowing in the outputs as well, something to watch I think.
Thanks for your reply. I already went ahead and replaced the 26.7K resistor with a trimpot and can readily adjust the DC offset. What I discovered however is that while this a wonderful sounding amplifier it is also a lesson in how not to design an amplifier. I think this amplifier was designed around the time DC amps were getting popular and effect of capacitors on sound quality was becoming more mainstream. It is supposed to be a minimalist design using all military grade parts. This design contains no capacitors at all in the signal path including the feedback loop. Most modern amp designs place a capacitor in the feedback loop to get unity gain at DC. What I found with the Mission 777 is that DC offset is a effected by the input source impedance. So there is no one correct setting. When I trim the DC offset to zero with the inputs grounded, I get 1.96 VDC with the Mission 776 preamp connected(250 ohms output impedance). I get different offsets with different preamps connected. So everything is a compromise. I finally decided to set the offset to zero with the Mission 776 connected because that's how I plan to use it.
While I had the amp apart I added snubbers to the rectifier bridges. I was going to replace the 220uF decoupling caps on the mainboard. They are high grade Elna caps but they are almost 20 years old. To my surprise every electrolytic cap measured completely within spec. I guess there is good reason to use military grade parts.
While I had the amp apart I added snubbers to the rectifier bridges. I was going to replace the 220uF decoupling caps on the mainboard. They are high grade Elna caps but they are almost 20 years old. To my surprise every electrolytic cap measured completely within spec. I guess there is good reason to use military grade parts.
Why don't you add an input coupling cap, in the real world it's going to have zero audible impact if you choose correctly. This configuration is inverting, and the input cap would reduce gain to zero at DC, not unity. The input impedance is pretty low R15 +16. This would let you set it up regardless of source impedance. Might be worth a try.
I still think with two trim pots you might get it better, the offset should not depend on source impedance. Maybe this is the way to adjust it, zero whether open or shorted. I was thinking along the lines of drift with temperature. Dont stray to far from the values as they are though.
Regards Karl
Edit minimalist design, maximalist price no doubt
I still think with two trim pots you might get it better, the offset should not depend on source impedance. Maybe this is the way to adjust it, zero whether open or shorted. I was thinking along the lines of drift with temperature. Dont stray to far from the values as they are though.
Regards Karl
Edit minimalist design, maximalist price no doubt
The whole mainboard module is about 3.5" x6" and is fully stuffed. There is no where to put an extra part unless you're willing to mount it free floating on top of the board. Anyways the offset is now near zero when it is in use and it appears to be quite temperature stable so I don't really care to do too much more to it. The modules connect to the power supply and the inputs via quick connect plugs. I think my next project will be to build my own amp modules using the same type of quick connections and just swap them in.
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