Sugden P128

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#42
Took me a while yesterday to work out what QC means.
Had the same problem this afternoon, only it took even longer.

Use Ib instead.

I = current
b subscript is where it is, i.e. bias.
Iq would be quiescent current


Check bias and output offset with the input shorted and no external load on the speaker terminals.With the transformers at the top and the 5534 at the bottom:
The lower left most should be Iq adjustment
The VR towards the middle should be the offset.
Attach voltmeters to each test point and check what is happening, as you make the adjustment.

Let the amp thoroughly warm up and recheck both periodically.
Might even be worth blowing a hair dryer over the four sinks to simulate exceptional Summer ambient.
 
#43
On the board underneath where the speaker cables join the board one of the leads also had a thin green wire going somewhere else on the board, is the green wire the feedback loop or earth link ?
Kind regards
Danny
 
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#47
Do you have any of the P128 schematics yet?
The feedback is shown in there.

The 5534 has it's own gain (A1) set by it's feedback.
The main amplifier also has it's own gain (A2), again set by it's feedback.

The total gain is A1 * A2 less whatever attenuation is built into the input components.eg, if the input resistor part of the RF attenuation were 1ko and the shunting resistor setting the Rin value were 100k, then the output from this attenuator would be 100/101. Or ~99% gets passed to the amp input.
 
#48
I have 1 schematic that you did on computer it think, well it's signed by you Andrew! But that's all I can find, can't find Sugdens diagram or your hand drawn?? Don't think am looking in the right place maybe!?
Also as I mentioned before my amps don't have the 5534 ic anymore as the amps where modded by Avondale audio
Regards
Danny
 
#49
I feel bad keep asking all these questions of you Andrew, it don't matter, I will just struggle on, on my own! I have got this far, am sure they will be fine!! They work fine just got some last things to install and setup!!
Will let you know how I get on!

Sat on Orkney board with nothing to do but wait for the boat back at 5pm
 
#50
The thin green wire on the underside connects the front end ground to the Main Audio Ground.
However that long thin trace carries both Signal Grounds and Decoupling Grounds (=bad).
Note that the bolt through the PCB has one side connected to the PSU Zero Volts and the other side connected to the MAG.
The 1.6mm of bolt shank is the short link from 0V to MAG.
 
#51
A2 gain is set by R24 27k and R15 1k0 giving 28times (+28.9dB)
A1 gain was set by R5 2k7 and R6 2k7 giving 2 times (+6dB)

At both the input and the coupling between stages the Rseries follows the Rin setting and thus 100% of the Signal passes (= no attenuation).

Your amp may have only A2 gain, i.e. +28.9dB
The 27k is between the relay and the IC.
The 1k0 is closer to the IC next to the group of 47uF Tantalum+Zener+100pF
 
#53
Ah MAG, main audio ground!!
I take it when this amp was built star earthing wasn't about then!? Haha
I want to get the best out of these P128's now known as my baby's lol
All thought and best things to do greatly appreciated as always
Regards
Danny
Ps thanks again to Andrew the man with all the know how 🙂
 
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