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Connecting RG1/RG2 to GND pad
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naa3e5  
#21 Posted : Thursday, April 9, 2009 9:40:47 AM(UTC)
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So I have a quick "learning" style question. How did you discover this oscillation? was it audible? Or could one find it only it they had a 'scope? I am reading all the different posts and building up my DIY knowledge as I am new to this hobby and found this scenario interesting and want to learn soething from it. I don't own a scope and my electronics knowledge probeably wouldn't lend itself to the correct employmtn of said 'scope either! So I am trying to figure out how everyone troubleshoots issues!
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Russ White  
#22 Posted : Thursday, April 9, 2009 9:40:51 AM(UTC)
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HI Ross, if you offset is that high would use output caps to protect your amplifier/speakers.
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krgaunt  
#23 Posted : Thursday, April 9, 2009 4:09:11 PM(UTC)
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Hi Naa3e5

I learned about the oscillation by using a scope. In this case the oscillation was in the MHz region, so I would not have heard it, but my power amp would have noticed it had I hooked it up. I don't know what the Sympatico cut-off freq is, but it may have passed the oscillation on to the speakers and fried my tweeters. This would not have been good, I have great affection for my tweeters.

With any new circuit that I construct, I take a number of precautions before connecting it to anything. First time power is applied I hold my hand on the board to see if anything is getting hot, then I measure input/output voltages with a meter, then I put a scope on it. If that's okay, only then do I connect the input terminals and check everything out again. It was at this point that I discovered the oscillation. The CP board is fine stand alone. Once I get the feedback under control, I'll let these boards cook for a few hours and I'll check them again. I'll put some test sine waves (400Hz, 1KHz, 19KHz) through the Buffalo and look at the CP output with the scope again. I want to check for noise and clipping. Once I'm satisfied the CPs are good, then I'll finally connect it to the power amp.
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krgaunt  
#24 Posted : Thursday, April 9, 2009 5:41:23 PM(UTC)
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Naa3e5,

One thing I need to say about placing my hand on the board. I have over 35 years of experience as a professional in the electronics and computing field. So I know when it's safe to put my hand on a board. If you are just starting out, you should not try this.

I can touch a low voltage board like the CP with no problem--I do have to be sure not to be wearing any metal on my hand that could cause a short. When I was repairing tube-type TV sets back in the 70's, I wouldn't touch the electronics when it was turned on. TVs have voltages up to 30k volts for the CRT and high amperage current for the deflection circuits. So safety is important. Even low voltage, high current power supplies (in power amps for example) can put out a lot of current and if I were to get a ring across the terminals I could cause a large burn on my finger. So don't touch circuitry that's energized until you have a great deal more experience.

Ross
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AudioBear  
#25 Posted : Friday, April 10, 2009 7:54:37 AM(UTC)
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krgaunt wrote:
Hi Naa3e5
...With any new circuit that I construct, I take a number of precautions before connecting it to anything. ....


I could not agree more Ross. I have even more degrees of testing :-) When I'm happy it is all OK electronically, with no overheating/dc/noise etc it goes into my test amp, a little T-amp connected to some rubbish speakers (just speakers no cases).

After this has run for a while and I'm happy, it gets connected to my better chip amp and some small KEF speakers. This allows me to check out the sound quality (to some extent) and to do a bit of stress testing.

If, after that, all is still well, only then does it go into the 300B valve monoblocks and my beloved Beauhorns for some proper evaluation ;-)

p.s. How are you getting on with your Counterpoints?

Edited by user Friday, April 10, 2009 7:55:51 AM(UTC)  | Reason: Not specified
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krgaunt  
#26 Posted : Friday, April 10, 2009 10:19:12 AM(UTC)
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AudioBear
p.s. How are you getting on with your Counterpoints?[/quote wrote:


I didn't get much time to work on the CP last night. I found the 4:1 S/PDIF mux card was putting a major 384kHz (2 x 192KHz) spike onto the ground and +12 wires. So I need to suppress that.

I have one CP that's not oscillating but with 16 mV and 56 mV DC on the output lines. The other board I can't stop from oscillating. I have 1nf on CC4 & CC5. As I increase the capacitance the oscillation frequency goes up. It's forming a beautiful 4.6 MHz sine wave now. I forget the exact amplitude, but it's in the hundereds of milivolts.

I haven't messed with discrete differential amps for a couple decades, so I'm quite rusty. I have to brush up on these circuits so I can figure out how to reduce the feedback. Back in school we had curve tracer analyzers which allowed me to match transistors. But these days I only have a scope to work with. If I can't get the CP's behaving in a week or so, I may just chuck them and go back to the IVY board. I got them as a curiosity anyway--something to fool around with until I can get a Buffalo-32.

RossG
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Lennert  
#27 Posted : Friday, April 10, 2009 11:39:14 AM(UTC)
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Ehmm... for someone without a scope... I just finished building my counteproints yesterday, but is it save to hook-up them up at this point or not? Think
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Russ White  
#28 Posted : Friday, April 10, 2009 11:49:10 AM(UTC)
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Hi Ross,

I am not seeing the oscillation you are, but here are a couple other things you might try.

Make R11,R12 221R.

Also try increasing the value of CC3.

The offset is not unexpected, but a bit higher than I am getting. I only see about 2mv.

Cheers!
Russ
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Russ White  
#29 Posted : Friday, April 10, 2009 11:50:39 AM(UTC)
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Lennert wrote:
Ehmm... for someone without a scope... I just finished building my counteproints yesterday, but is it save to hook-up them up at this point or not? Think


The best thing to do if you don't have a scope is to use your DMM and at least check that the DC offset if reasonable. If it is then try the cct with a cheap amp and disposable speakers.

Cheers!
Russ
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NicMac  
#30 Posted : Sunday, April 12, 2009 8:41:34 AM(UTC)
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In the end I have on channel working like a dream, but only hum from the second.
As I like stereo I will try to build a second set of boards hoping that this will yield 3 functional boards in the end...

Nic
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sp502  
#31 Posted : Sunday, April 12, 2009 10:34:40 AM(UTC)
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Yesterday I finished the one board of the Counterpoint set, and with the use of my scope
I have seen some interesting points.
First is that with the use of 14VA - 12V transformers and LCBPS, if the LCBPS is configured
in any voltage above +/- 9V the CP will throw it down to about 9.2V (only one channel connected).
Second point is that then there exists a small difference in the absolute value of the voltage
that makes CP unstable and produces 50Hz(Europe's mains frequency) hum in the outputs.
The cure is to bring down the absolute value of voltage of LCBPS to 9V with accuracy of about
0.02V. Then there is only music.. :d/

The high frequency oscillation that was mentioned in a previous post, shows up (a very low signal) but with
the use of better probes almost disappears, so I assume that the probes/scope affect the output
of the counterpoint and maybe there is no such signal there under normal conditions.

By the way, it sounds fantastic!

PS Is LCBPS not enough for Counterpoint, should we use something else?
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sp502  
#32 Posted : Monday, April 13, 2009 1:47:26 PM(UTC)
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Investigation continued...

After completing the second board, and experimenting with the power supply
I have come to this conclusion:
Counterpoint draws a lot of current!

Furthermore to be at least sure that it works properly +-12V are the minimum,
as at lower voltages the relay on the output becomes unstable and may do nasty things,
from noise to loud clicks & plops.

In few words I am currently using 2x(12V, 14VA) transformers in a single LCBPS,
one for the positive and one for the negative leg. The transformers may have a 12V
nominal voltage, but without load they give 18V... Even with that specs LCBS outputs
at best +-12.2V with the Counterpoint set.

With two channels two 14VA transformers can't give more than 12V volts in the LCBPS' outputs!

I don't know if I have missed something, but how much is enough?
What are the power supply requirements for the Counterpoint?
Shouldn't be a warning that the usual transformer(15VA) used for the rest of the TP's boards is not adequate?

As for the results...
With only one channel everything is great, dead quiet with no signal.
With two channels there is a very low HF noise and LF hum (must be very close to speaker to hear it).
Maybe the 15V are the right point for nirvana...
Although there is still one step to get to the best setup point, I can tell that the sound is just magical Drool


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Russ White  
#33 Posted : Monday, April 13, 2009 1:59:43 PM(UTC)
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Yes, you may need to jumper R1-4 on the LCBPS because of the high current demand from the counterpoint.

Each counterpoint draw about ~120ma from each rail, depending on your rail voltages. So plan your trafo accordingly. :)

Cheers!
Russ

Edited by user Monday, April 13, 2009 2:00:26 PM(UTC)  | Reason: Not specified
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sp502  
#34 Posted : Monday, April 13, 2009 3:09:35 PM(UTC)
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Russ White wrote:
Yes, you may need to jumper R1-4 on the LCBPS because of the high current demand from the counterpoint.

Each counterpoint draw about ~120ma from each rail, depending on your rail voltages. So plan your trafo accordingly. :)

Cheers!
Russ


That's the trick!
Now I have solid +-15V and the sound's gone to another level...
Tomorrow I'll give it a try with one transformer.

Thanks Russ!

PS. I think it's time for a Counterpoint manual to be published.
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francismorrin  
#35 Posted : Monday, April 13, 2009 4:10:32 PM(UTC)
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Hi Russ/Brian,

I received my counterpoint kit a week or 2 ago and started sodlering up tonight. I'm missing one R27 (ie 75R) resistor. I thought I had some lying around, but can't find them. Would some 82R be ok instead? - I have some of those in 1% MF.




Fran
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Brian Donegan  
#36 Posted : Monday, April 13, 2009 4:28:17 PM(UTC)
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If you can stick it out long enough, I'll send you the proper value.

(Hey, wow, that could be taken the wrong way... meant "waiting" :) )
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krgaunt  
#37 Posted : Monday, April 13, 2009 5:02:43 PM(UTC)
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I think I found the reason for my CP oscillation problems, you know the 600mV 2.5 MHz oscillation that I've been getting. Turns out the voltage divider (where R15, R16, R28, and R29 join hands) is at -2.0 volts (not a typo). Yes, a NEGATIVE voltage. AVCC is +3.46 V, but the op-amp side of R28 and R29 are both at - 2.3 volts. On my other board the voltage divider is at +1.76 volts (normal). I poped out the op-amp, and the voltage divider now is at +1.76 volts. So I have a bad IC that's causing these problems.

RossG
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francismorrin  
#38 Posted : Monday, April 13, 2009 5:33:22 PM(UTC)
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Thanks Brian,

but wait a bit - I'll seach a bit more, I really thought I had some 75R in the workshop. I'll look more tomorrow.

Fran
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Russ White  
#39 Posted : Monday, April 13, 2009 8:21:34 PM(UTC)
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Ahhh, well that makes much more sense. :) Good Work Ross.
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Russ White  
#40 Posted : Monday, April 13, 2009 8:22:17 PM(UTC)
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Fran it should be ok even with the 82R if you are in a hurry.
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