Best way to install Superreg in a preamp

[hacker]

Great stuff here folks - thanks for all the input. I'm sure this will be useful to more people than just me.

One further thing that I've noticed is Andy's statement in the documentation which says:

...care has to be observed with the load presented to the regulator by the ciruit being powered. Specifically, small film or ceramic capacitors close to the regulator will almost guarantee instability and oscillation.

Page 11, section 9.4 (Performance Verification)

I'm going to be using the SR to power two gain stages. Each of these has the power rail decoupled by 56uf oscons. The power rail to each gain stage is, however, fed via a 27R resistor. Does this mitigate the above quote?

Further, as I'll only be running ~19v into the 27R, should I swap it for a lower value? For example (and here my lack of knowledge of electronics starts to show):

At the moment, voltage across the 27R is 24.4v, therefore I assume there is a voltage drop before the gain circuit, which must have less than 24.4v across it. I understand the 27R is there to present a load to the PSU to draw a specific current. If I'm going to put ~19v across the 27R, I'm changing 2 things: the current draw and the voltage across the gain stage. Should I change the 27R for a new value? Remove it entirely? Perhaps measure the voltage across the gain stage after the 27R and match it accordingly? Help!

This sure is a learning process!

Cheers,
Carl

 

[bivalve]

Carl,

An OSCON is a form of electrolytic cap, so no problems with oscillation.
The 27R resistor and 47uF elcap act together as a filter, called an RC filter. This one has a half power frequency of 125Hz. Below 125Hz power comes through the resistor, above 125Hz the cap is increasingly able to supply power better than the resistor.

This is pretty amazing really. For all the supercaps/superregs etc we might have on the other end of the resistor, the cap supplies power for most of the frequency range. This is why the quality of the cap is so important.

The change in proportion of current through the resistor vs from the cap is gradual, and at 3 times the half power frequency (375Hz) is down to ~1% from the resistor. The cap also begins to supply current below 125Hz.

The effect is to keep out noise and ripple above 125Hz, and to stop one gain board etc from modulating another. The filter is a compromise. It barely affects 100Hz hum, but lowering the frequency requires bigger caps and/or a greater voltage drop across the resistor.

The 27R resistor is not there to present a load. It would if it were connected to 0V, but here the circuitry beyond it is a much higher resistance.

Remember V=IR, and a Gain board draws 10mA, so the voltage drop across the resistor is 10x10-3 x 27 volts or 0.27V.
The equivalent resistance at DC of the gain board is (R = V/I) 24/(10x10-3) or 2400R.

I.E. you don't need to change the 27R resistor. Its also not there to draw a specific current.

The fact is than separate power supplies for each gain stage sounds a lot better. We might guess that the two circuits can no longer modulate each other at lower frequencies. When I changed over to two SRs I removed the 27R resistor so that the each SR could get a good grip on its gain board. I'm sure they are much better filters than the RC circuit. And what's the point in putting a 27R resistor in series with your lovely microohm output impedance regulator.

I also removed the elcap, thinking that since it was in parallel with the elcap on the SR (C5) and my SRs are close t0 their gain boards, it was redundant. I might experiment a bit more. Some people leave both in in fact.

Regards,
David

 

[Jo Sharp]

David,

You might consider the effects on the circuit being supplied of having a very low impedance, 'fast' power supply....if that circuit has a tendency to 'rave off', as Les W so nicely puts it, then a faster supply can make things worse.

 

[bivalve]

Thanks Joe, I'll take heed. I have already run into strife. See 'For those who want Silmics #75'.

I'm keen to put my understanding of things down at least once, to see if it stands up to scrutiny.

Regards,
David

 

[hacker]

David, thanks very much for the insight into the use of resistors/capacitors in these circuits - it clears things up a bit.

Here's some pics: http://haggis.kicks-ass.net/audio/ (see the link to fitting a superreg).

I've got it all working and will write it up properly tomorrow (I'm knackered - it's 4:40am!!!).

Basically, I've modded the 140 and taken out its LM317. DC out is now 37v. I used two 100R in parallel on its DC output to cut the voltage down to just under 32v for the preamp. The 32v is fed into the SR in the preamp which is setup to produce 24.9v with TPR and remote sensing. I partially rewired the 62 and put new 4/5 pin din sockets on the 62 and 140 while I was at it.

Only one small hitch - when I turn the 140 on I get a loud, short, sharp blast of high-pitched sound through the speakers. It's kinda like someone doing one of those mega-loud whistles when they put their fingers in their mouth and blow. It lasts for maybe a quarter second and stops. After that the caps do their usual buzzing until everything sorts itself out.. then it plays music Can't really crank it up or the neighbours'd go daft, but at least it works, kinda.

Any idea what the high-pitched blast is? It can't be good for the speakers and must indicate a problem somewhere, but I'm not sure where... could it be the SR? Or perhaps because I've removed the 140's LM317? 0v issues? Damn, I'm tired and off to bed; perhaps one of you kind gentlemen will have suggested an answer by the time I get up

Cheers,
Carl

edited: I wrote 28.9v instead of 24.9v

 

[bivalve]

Carl,

Too excited to sleep!

It all looks pretty good. Congratulations. Much more fun than simply buying something.

Re your small hitch, i've no real idea what the cause might be. It is however a dreaded oscillation.

You have one small piece of non-optimal wiring, however. The 0V connection on the SR should go to the star earth, not the power input socket. You already have a 0V connection between the socket and the star earth. All other 0V lines reference the star earth.

You might try putting a 0.1uF film cap between TRIN and TRGND on the SR. Check the online data sheet at the website of your prereg IC's manufacturer.

For interest what type of cap are you using for C5 on the SR?

 

[Patrick Dixon]

A question for Jo:

On another thread you mentioned that you were using 1R in parallel with a small inductor with local SRegs - I wondered what value the 'L' is, and how did you made it?

I'm currently using a (non-local) SReg in a modded Squeezebox with just a series 22R in the SB. I found it needed the 22R since without it I had oscillation on the PSU rail at (IIRC) about 100KHz.

 

[hacker]

It all looks pretty good. Congratulations. Much more fun than simply buying something.

Hell yes! It was a long night, but fun

Re your small hitch, i've no real idea what the cause might be. It is however a dreaded oscillation.

Aha. I did wonder how this manifested itself and if it was noticable without a 'scope. In fact, does this mean that oscillation is a temporary thing, and settles withing the first second? Or does it mean that the circuit is inherently unstable and must be sorted before damage occurs to my 62?

You have one small piece of non-optimal wiring, however. The 0V connection on the SR should go to the star earth, not the power input socket. You already have a 0V connection between the socket and the star earth. All other 0V lines reference the star earth.

I'll change that right now (well, after a coffee - I've only had 4 hours sleep

You might try putting a 0.1uF film cap between TRIN and TRGND on the SR. Check the online data sheet at the website of your prereg IC's manufacturer.
[/QUOTE

Hmm, I don't have one. I'll have to nip out to Maplin. Any recommendations?

For interest what type of cap are you using for C5 on the SR?

C5 and C6 are not to be fitted, according to the build instructions that andy sent me. Well, according to some sections of them. The diagrams still have a 100u/35v cap on them, but the parts list says 'not fitted. Hmm. I've got a 100u/35v rubycon kicking around, is it worth trying? Perhaps this could be the source of the oscillation?

Cheers,
Carl

 

[bivalve]

Carl,

C3 and C5 are in parallel, so have you something in C3 position?

The 0.1uF cap is not critical in size. Anything between 0.01 and 1uF would do for diagnosis. It should be a film cap, however.

Must return to dinner.

David

 

[hacker]

Carl,

C3 and C5 are in parallel, so have you something in C3 position?

The 0.1uF cap is not critical in size. Anything between 0.01 and 1uF would do for diagnosis. It should be a film cap, however.

Must return to dinner.

David

Yes, I've got a 22uf Rubycon ZA as C3.

Dinner? What timezone are you in?

Cheers,
Carl (off to Maplin we go...)

 

[hacker]

Ok, back from Maplin. I put a 0.1uf film Wima cap across TRIN and TRGND and it made absolutely no difference

Only so much time before my girlfriend comes back from Newcastle and I can't have my shit sprawled all over the dining room table when she comes back or there'll be trouble

 

[trancera]

Carl,

Where abouts are you?

When you write up the 317 removal and SR addition can you make it nice and detailed for idiots like me. My 140 and 62 are a little nervous at the moment

Also. Have you turned on the 140 without the 62. Does it still squeel?
If not are you sure the relay is working correctly?
Just a couple of thoughts.

Brian.

 

[bivalve]

Carl,

I'm at GMT+10 (Melbourne, Australia) and I'm already into the post dinner Cognac.

First try a lucky shot. Power up your Power amp w.o. it being connected to 62. Same noise, then problem is in the power amp. Slight hum is normal.

Next reconnect and put 27R resistor in SR-pre connection. Try +ve sense either end of resistor. No improvement then...

Disconnect Gain stage outputs from output socket and connect output socket signal pins to earth. Any effect?

If not, reconnect and power delay circuit direct from power input socket. Remove power coming from SR somehow. What happened?

What you describe is typical of a circuit settling at turn-on. Probably the circuit is unstable until a big slow elcap is charged.

Fault finding is an art. Lets hope some other artists have a look-in. The order of the above is not critical.

David

 

[hacker]

Carl,

Where abouts are you?

I'm in Hull (for past sins, I'm sure). Where are you? If you're close it would be great to do a side by side comparison of 62/140s. What mods have you done?

When you write up the 317 removal and SR addition can you make it nice and detailed for idiots like me. My 140 and 62 are a little nervous at the moment

I'm feeling a bit of an idiot right now - I don't like it when I don't know what's wrong! But I will write it up in good detail, sure.

Also. Have you turned on the 140 without the 62. Does it still squeel?
If not are you sure the relay is working correctly?
Just a couple of thoughts.

I just tried it and there was no squeal, so it must be something in the preamp circuit that's causing it. The relay would be a likely candidate, but I've not changed anything in the relay circuit apart from the input voltage (24.4v to 24.8v).

Damn, this is so frustrating!

Carl

 

[bivalve]

Carl,

Forget the re insertion of the 27R resistor. Its still there, I know.

David

 

[hacker]

Carl,

I'm at GMT+10 (Melbourne, Australia) and I'm already into the post dinner Cognac.

Aha, I'm in UK so that would explain why you were posting at 4am/7am this morning!

First try a lucky shot. Power up your Power amp w.o. it being connected to 62. Same noise, then problem is in the power amp. Slight hum is normal.

I've just done this and there was no squeal.

Next reconnect and put 27R resistor in SR-pre connection. Try +ve sense either end of resistor. No improvement then...

I've changed from remote sense to local, so this is a moot point now.

Disconnect Gain stage outputs from output socket and connect output socket signal pins to earth. Any effect?

Um, why? That sounds a bit wierd... what could it tell me?

If not, reconnect and power delay circuit direct from power input socket. Remove power coming from SR somehow. What happened?

I'd like to try that, but must first check the operating parameters of the relay chip. I don't want to fry it with 32v!

Edit: the PDF spec file is here: www.acoustica.org.uk/t/naim/preamp_pics/ultra_preamp/datasheets/prma2a12.pdf

I'm not sure how to read it. Page 4 says that the max switching voltage for a PRMA2A12 is 200v, but page 5 says the max 'coil voltage' is 21v. Does that mean I can put 32v into it or not???

Fault finding is an art. Lets hope some other artists have a look-in.

Amen! Thanks for all your input, you've been a great help.

Carl

 

[trancera]

Aberdeen, I was hoping there was a closer Haggis connection!

Yeah and I could bring the other 140 and the other and the 42.5, 42 ....

It'll be a simple problem, but as always a to find

 

[hacker]

Aberdeen, I was hoping there was a closer Haggis connection!

Ah, there is - I'm originally an Edinburgh lad. In fact, we're up in Aberdeen on the 27/28th of this month for our anniversary. We'll probably visit Stonehaven along the way. It's probably best that I don't mention I'd like to stop off for a few hours to listen to hifi...

Yeah and I could bring the other 140 and the other and the 42.5, 42 ....

It'll be a simple problem, but as always a to find

Yeah, my money is on the relay at the moment. Maybe it's over voltage or something. Anyway, it's all switched on with the lids on at the moment but I hope to get inside it again soon.

Carl

 

[trancera]

Yeah best not mention it. Even approaching my place you'd get a slap - Sevenoaks is a couple of hundred yards away - sometimes go in there for a laugh

Considering Tri amping tomorrow, but I find the cabling sooo messy (and stressful) to set up!

Decorating soon so might do some taking off of skirting etc to add power and speaker cables.

 

[Jo Sharp]

A question for Jo:

On another thread you mentioned that you were using 1R in parallel with a small inductor with local SRegs - I wondered what value the 'L' is, and how did you made it?

I'm currently using a (non-local) SReg in a modded Squeezebox with just a series 22R in the SB. I found it needed the 22R since without it I had oscillation on the PSU rail at (IIRC) about 100KHz.

Patrick,

The little inductor was recommended by Andy Weekes for use on the output of a Superreg that is feeding a circuit using SMRs as de-coupling caps. Don't know the inductance...but...

It is made up with a coil of 20 turns round an 8mm former (I used a drill bit); remove from former and solder into place i.e. air cored. For the wire I stripped an old transformer so the wire is enamel insulated and stiff enough not to unravel when formed.

Hope this helps.