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Old 20th August 2005, 07:57 PM   #541
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Default Progress

I've gotten my RevA running again today. I needed to improve the fit of the chips on the heatsinks.
I found they needed what I consider excessive gain to drive them.
With max level from my pre-amp they were at normal listening levels. This drive level was above what I normally use with a 35 w/ch stereo reciever and 6db boost added through a Marchand active x-over. I took the x-over out of the loop and got the same results. Taking the x-over out also left a low-level hum in the system.
So, I'm going to check my resistor values, starting with the gain stages. I don't know where to start with chasing the hum away. We'll try one thing at a time. Suggestions are welcome.
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Old 20th August 2005, 08:10 PM   #542
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Quote:
Originally posted by maf_au
Ok, that's it from me, it's starting to come together. See pic.

Toroid cabling check question:

I have this toroid:

http://www.altronics.com.au/index.as...=item&id=M5525

On the My_Ref PCB, there are 4 connections:

PL3 Gnd
PL4 Gnd
PL5 24va1
PL6 24va2

The toroid secondaries are paired Red & Black, White & Orange.

I see that PL4 and PL5 (Gnd) are on the same PCB trace. Should I connect:

Red = PL5
White = PL3
Black PL4
Orange PL6

Thanks,

Michael
That will work fine. As long as red and orange are connected to the ac inputs, the wire and black is not critical. On this circuit anyway.
I see what those cost. Even corrected for Aussie dollars to UDS, they are still not cheap. For US, I had some custom made in Atlanta at Victoria Magnetics that are 60 - 65.00 for a 300 watt, toriod. These have electrostatic shields to boot. I think they are better than the hallowed Plitron.
But bang for the buck, the 48 volt centertapped ones from PE, are unbeatable. Some experts feel that E core is better than toriod. The bandwidth of the toriod lets more mains garbage through.
Your amp is looking good!

George
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Old 20th August 2005, 08:21 PM   #543
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Default Re: Progress

Quote:
Originally posted by bg40403
I've gotten my RevA running again today. I needed to improve the fit of the chips on the heatsinks.
I found they needed what I consider excessive gain to drive them.
With max level from my pre-amp they were at normal listening levels. This drive level was above what I normally use with a 35 w/ch stereo reciever and 6db boost added through a Marchand active x-over. I took the x-over out of the loop and got the same results. Taking the x-over out also left a low-level hum in the system.
So, I'm going to check my resistor values, starting with the gain stages. I don't know where to start with chasing the hum away. We'll try one thing at a time. Suggestions are welcome.

Very curious, as the gain on my REV A actually seems on the high side. I actually have to go with lower volumes compared to my "Inverting NOT" amp...

If you are getting hum it is most likely coming from you mains GND. Just as a test try disconnecting it from the REV A.

I am sorry I am not more help... Hard to concentrate when 2-3lb small mouth bass are waiting outside your back porch.

Back to the creek...

I will be thinking it over though...

Cheers!
Russ
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Old 20th August 2005, 08:24 PM   #544
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Default Re: Progress

Quote:
Originally posted by bg40403
I've gotten my RevA running again today. I needed to improve the fit of the chips on the heatsinks.
I found they needed what I consider excessive gain to drive them.
With max level from my pre-amp they were at normal listening levels. This drive level was above what I normally use with a 35 w/ch stereo reciever and 6db boost added through a Marchand active x-over. I took the x-over out of the loop and got the same results. Taking the x-over out also left a low-level hum in the system.
So, I'm going to check my resistor values, starting with the gain stages. I don't know where to start with chasing the hum away. We'll try one thing at a time. Suggestions are welcome.
Since it works, it should be easy to find. Check ac input, should be 24 - 25 volts. Then DC on each channel, 34 - 35 volts on each rail.
Check dc offset for each channel, mine is 10-15 mv, some report lower. If it is mv level you are fine.
Since the LM3886 is unity gain, the LM318 provides all the gain. The ratio of the 100k input resistor is important, along with the 12K feedback resistor.
Also recheck what you have for small film caps. I almost strapped a 100 nf cap around the 318 instead of the required 100 pf. Think this would have created some issues.
The hum makes me think there is a grounding issue. This may be the gain issue also. Check ground from the body of the RCA to the power supply ground. Should be 1 ohm. Check the RCA from center to body, should read 100K. Check RCA center pin to input film cap, should read 3.3K If these all check the input section is right, try looking over the feedback loop. Maybe polarity of the electrolytic.

George
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Old 20th August 2005, 11:00 PM   #545
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Panelhead, Thank You

I started by checking for ground issues. Right off the bat I found something puzzling about my input resistance readings. I crossed my input leads! I was confused by what I thought (I ***umed) was a "minus" sign at the input...Nope, looks like the numeral "1" now...Oh well.
I'm switching the leads and will do some checks, then back to the speakers.
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Old 20th August 2005, 11:29 PM   #546
maf_au is offline maf_au  Australia
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Russ, George:

Thanks for the help and encouragement, if it sounds as good as it looks I'll be happy

There seems to be no such thing as a good, cheap transformer here in Australia. Shipping them from the US would add a lot of freight, and risk of damage because of their weight.

Unfortunately, I'm running out of time to finish it today, gotta hop on a plane in a few hours... Looks like I'll have to finish it Friday night.

Michael
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Old 20th August 2005, 11:35 PM   #547
maf_au is offline maf_au  Australia
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Russ,

With the monoblocks, is the pcb sized such that a pair of them can be attached to a heatsink that has holes drilled to suit the My_REF?

If not, please let me know what the minimum hole-to-hole spacing is and I'll predrill extra holes to suit, I'm sure I have adequate width...

I'm thinking it would make testing them quick if I could swap them into the same case. I've got 12cm from heatsink to case divider.

Michael
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Old 20th August 2005, 11:36 PM   #548
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Quote:
Originally posted by maf_au
Russ,

With the monoblocks, is the pcb sized such that a pair of them can be attached to a heatsink that has holes drilled to suit the My_REF?

If not, please let me know what the minimum hole-to-hole spacing is and I'll predrill extra holes to suit, I'm sure I have adequate width...

I'm thinking it would make testing them quick if I could swap them into the same case. I've got 12cm from heatsink to case divider.

Michael

Yes.
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Old 21st August 2005, 12:16 AM   #549
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Default Go figure

Switching the input leads did solve the gain problem.
The hum remains.

Some measurements: R/L
ACV secondary 25.2/25.17
DCV rails 32.4/32.4
DC offset mV 6/7

I need to get better access to the input to measure and inspect solder joints. Some disassembly comes next.

RCA body to ground, ohms 0.4/1.4
RCA pin to body NR/NR

Those are suspect and I'll also check across the input cap. I had the hum with either input connected. It is on both channels.
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Old 21st August 2005, 01:00 AM   #550
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HI Ed,

Your power measuerments are nearly identical to mine, including offset.

What, is odd is that my resistance from RCA body to GND is exactly 1ohm. I suspect your mains GND and pwer GND are connected. Am I correct? Either at the RCA or some other place. If so that could be your hum.

Cheers!
Russ
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