First class-a project: ESP P36

[ultrachrome]

ESP project 36. Was supposed to be my first amp but parts sourcing was too much for me at the time.

Came across this chassis at work, had a transformer without a home, already had most of the parts so about a month later, music!

A few outstanding tasks: need to clean up the wiring and the "heatsinks" that I'm using on the TIP2955s in the power supply don't seem sufficient as they are cooking at 140F.

[Stocker]

140F is only cooking for you. The semiconductors should be just fine. If it were 140C, you would have some problems.

[ultrachrome]

I have about 3hrs on this amp and I'm not feeling the love. Been searching older DoZ posts and I'm seeing much happier people.

The treble is kind of, how you say, tizzy? Just not that smooth. My gf mentioned it immediately.

I'll recheck bias and voltage tomorrow and maybe put some more hours on it before swapping with an AKSA 55 for a sanity check.

I wasn't sure what voltage to set the output so I set it to 16.6V which is half what the power supply measures connected to the circuit. Should I bump it to match the no-load power supply voltage?

Current is set to 1.5A (+/- .05). While biasing the amp, everything was very stable and pretty easy to adjust and it stayed that way. I did not do the bias stabilisation modification.

Each channel has it's own cap multiplier fed by its own 25V secondary from a 400VA Plitron. Outputs are MJ15003s. Everything else is per the construction article.

Any ideas from fellow DoZers?

[DocLorren]

Hi Ultrachrome,

The output voltage should be half the loaded supply voltage so you are fine there. I do think you have underbiased the DoZ a bit.
Rod Elliott advised 1.7 Amp and my DoZ is very transparant in the high frequencies (coupled to FE108ES Buschhorns). Do double check for bias runaway though because those extra 0.2A are going to stress your heatsinks some more and you might have trouble keeping everything in hand then. Make sure the amp is stable (+/- 10mA) or at least three hours under the future work conditions.
Let us know what happens,

DocLorren

[ultrachrome]

I though Rod's article was suggesting lower bias due to lower supply voltage but now I realize I misread his statement:

adjust VR2 to obtain 1.7A (for a 40V supply and T0-3 transistors) or 1.4A for flat-pack transistors anda 30V supply

Since my supply is only 34.8V I thought I needed to keep the bias lower.

But it makes sense that if I'm using appropriate output devices and I have the current capacity, I should bias higher.

Thanks, I'll try it tonight.

[ultrachrome]

I'm a little confused about setting quiescent current. Before I used the meter's amp mode, connecting the meter between the suppy and circuit with all current flowing through the meter.

This time I'm measuring voltage across a 1ohm resistor. I have current sitting at 1.7A after about an hour of warm-up and adjustment. My RatShack IR thermometer reads 138F/59C.

What happens after I take those 1ohm resistors out? Is there an appreciable change in the current?

Is there another way I can monitor quiescent current without disconnecting +ve from the circuit?

[ultrachrome]

I'm having a heck of time with quiescent current.

After setting it up to 1.7A and ensuring it was stable for a couple of hours, I plugged my speakers in and the left channel was now louder than the right.

I have a separate supply for each channel and I had a pair of 1ohm 10W 10% resistors. I used one for each channel. Since I can't really measure these resistors accurately my quiescent current in both channels is probably different. Could this account for the channel imbalance?

If I set quiescent current for both channels with the same resistor, it would take hours, allowing for ample warm-up time. So there's really no way, that I know of, to spot check and see if the current is the same in both channels.

Right now I have the left channel powered up through a 1ohm resistor and after 20 minutes, q.current was 1.6A. After 40 minutes it was 1.65A. I'm about to check the right with the same resistor.

As far as general testing is concerned, I've got a scope that I can hardly use (no manual and no experience). Can someone recommend a good source of information for testing power supplies and amps? I feel like I'm flying blind here.

[ultrachrome]

It's clear that I am losing my mind.

Spent over an hour per channel setting quiescent current. Left it at 1.77A. Rearranged some of sound treatment that had been taken down to fit the projection screen.

Channel imbalance seems to be gone. Heatsinks are 60C. Amp sounds great.

[EchoWars]

I have a .1 ohm resistor permanantly in line from the power supply to the driver board specifically to monitor bias current.

Try one thing though...monitor the bias current at power up (and for the first 60 seconds or so). You may find that the current through the output devices exceeds (or is close) to the end of the SOA curve until C6 charges up and the output voltage is centered. Seems to be a design problem with the DoZ.

[AKSA]

Ultra,

How's the amp coming along now? How does it compare with the AKSA 55?

Cheers,

Hugh