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Is 18.6V enough for a 32.5 pre?

chiily

chiily

PFM Special Builder
The subject says it all, but I'm concerned that the voltage may be just too low for my 32.5.

I have two ALW's SR tucked away inside the little pre, dropping the voltage from my DIYCap (pre reg + reg) from 24V to 18.6 (ish). This gives me just over the 5V drop required for the SRs. The voltages appear very stable out of the DIYCap and SRs on my DVM. sadly I don't have a 'scope to check for any analogue signals.

You see, I'm a little disapointed with the sound. I seem to have lost the 'airiness' around the sound, the mid band is not as clear as it once was, quite a bit of background noise has appeared (this could be my aging cartridge). In fact music sounds a little flat.

I have trying shorting out the 27R resistor on the unity gain and final gain stages to not much effect sound wise, and it doesn't matter whether I use Naim stock MC cards or my modified cards (no zener diode, no reg resistors, 56uF SP across the rails), the result is just flat.

I wondered if the reduced voltage could be having any effect on the dynamics of the pre-amp, transistors not working in their proper operating zone (bias), etc.

I could up the voltage out of the DIYCap and the SRs, but it's an awful lot of fiddling and messing about. I just wondered if I would be heading the right direction.

Any thoughts or comments certainly appreciated.

I can provide more information to how the DIYCap is configured if required.

PS. I have removed the relay board too. Signal directly wired output from the final gain stage.
PPS. I'm running a 4 pin black SNAIC directly from the pre to the power and a 5 pin black SNAIC from the DIYCap to the pre to separate the power and signal.
PPPS. The DIYCap 0V is starred from its 5 pin DIN plug. The SR 0V are starred from the pre's star point too.
 
There's no reason why the Naim shouldn't operate on a low voltage other than the relay, which I note you've removed, just won't energise. I like to run the Naims on 30-32V and whilst this is politically incorrect, the sound stage is just so solid and the dynamics are well.......try it and see.
 
Les,

Thanks for the answer. Running at 30V would solve a good many of the complex regulator bits in the DIYCap, that is dropping back to just the one reg per rail. I read somewhere about over regulation of supplies.

However, I do like the sound of the OSCON SPs as rails caps on each board in the pre, so I'll propably stick to 25V, that is the max voltage of these caps.

I'll squeeze it back up to 25V.

Would you recomend leaving the zener diode cct off the MC boards too?

Thanks for the info.
 
I noticed the same thing when I put LT1086 regulators inside my NAC92 pre-amp, dropping the rails to 20V. There was something about the sound which I didnt like - it sounded constrained, uneasy, and it was only when I increased the rails back to 25V did the sound spring to life.

Although in theory, the preamps should work at a much lower voltage, in practice, to my ears it seemed like it was running out of dynamic headroom. I wonder if there's any truth in this because I noticed that the effect was worse with CD as input. As most CD players can output in theory up to 2V rms (from memory) or ~ 5.6V peak to peak, this is quite a large proportion of the available voltage swing. Garf, do you notice any difference between using CD or phono sources, which usually give lower voltage levels into the naim buffer stage.

Is this perhaps also the same reason why some people report better sound when using input attenuators on the CD inputs to their preamps? A popular explanation for why input attenuators work is that you are no longer working with the volume pot in its minimum position, something which I've never really been able to accept. It may just be that you're operating the input buffer stage well within its end stops by reducing siganl amplitude?

I'm probably talking rubbish but just a thought.

Les,

Do you know or anyone else know anything about the relay switching circuitry in the NAC92, i.e can I run it off 28V or greater, as you seem to have done?
Currently I'm still running the relay switching part of the preamp off 25V for fear of damage. I havent worked out the schematic, all I know is that the relays can work up to 44V. But as for the rest of the circuitry, I would imagine it's OK to operate at 28-30V but I'm not sure. It would just make my wiring a lot neater if I could run all the preamp circuitry off the same voltage.


John
 
You see, I'm a little disapointed with the sound. I seem to have lost the 'airiness' around the sound, the mid band is not as clear as it once was, quite a bit of background noise has appeared
Click to expand...

Garf,

I'm sorry to hear of your experience thus far.

Just a quick question, is the noise you mentioned simply more vinyl noise, or noise even when the preamp is idle. Neither are correct, but the latter may indicate instability somewhere, at least one 102 had this problem when using multiple reg's - easily cured by the addition of a small cap to each gain stage, IIRC.

As a suggestion though how about returning the pre to standard and putting the SR's in the external PSU?

There's a few others here running reg's internally in 72's etc, and a colleague of mine has something like 8 in his 102, all running at similar voltages to your and that's the best preamp I've ever heard, so I don't think whatever you're experiencing is insurmountable.

Drop me an email, I can send you something to try.

FWIW my NAC112 runs at 15V (by design) and works really well :)

Andy.
 
I didn't intend this post to be so long.

Jmy,

First off, I haven't really listened to the CD via the pre at all. Something I've certain thought about, time is always against me at the mo. It would help tracing the source of the noise down. I do believe I am going to have to push the voltage up to 25V too, to find out.

Confession time: I just don't like the sound of CD, thought I've never heard a top shelf player in my life. I sort of quit hifi in about 1993, long story but it involved a woman and her three kids :) I'm now back in the fold, skint hence the diy work and the only CD player I have is a second hand CD52SEII. Not a bad player in its day, but eclipsed by my LP12. Oh and I only own 10 CD's, six of these are 'I'm Sorry I haven't a Clue' :)

I did have a LT1086 running at about 20V in the pre at one stage, feeding just the phono side. That sounded pretty good, but I was messing with caps on the MC boards at the time, so it's hard to tell the real benefit. I don't remember quite so much noise though.

Thanks for your thoughts. I will try the CD player. I do have a tuner, NAD 4225 (soon to be mod'd), but only a sh*ty antenna so I get noise from that source.

Andy,

Funny thing is, I wandered past the longue(?) this morning and noticed the noise from about 12 foot away. Admittedly the volume control was at about noon (neighbours out shopping!).

I did wind the wick up (about 3 o'clock) with no source, only phono selected and listened to the tweeter. The noise sounds like faint spitting, crackling almost, accompanied by a constant hiss. Switching to tuner, not turned on, but plugged into the back of the pre, the volume of the hiss dropped but the same noise of spiltting, crackly was still there, both channels, though fainter. Non of these sounds are loud, the spitting and crackling are faint.

If I get the time this week, I'm going to China on Saturday for three weeks (holiday, not work, hence me being skint), I was going to wire my DIYCap 24V rails directly on to the rails of the pre, removing the SR's from the equation. In fact I was thinking of wiring a sub-minature DPDT switch onto the pre so I could switch between SRs in, SRs out. I'm trying to sell them to a mate of mine who thinks valves are the bee's kness, but he coming around to solid state. The issue I could see was that the SR's would still be wired to the 0V star point, unless I can find a TPDT switch.

Originally I had thought of putting the SR's in the DIYCap, but I was concerned about the 38V coming off the smoothing caps overvolting the SRs. I know it's only 2V, but aren't the caps on the SR rated at 35V?

One thing that has concerned me for a while is the heat generated by the 1086s in my DIYCap and to a lesser extent the 317's on the SR's, but only on the 'phono power rail supply. The regs go like this:

Smoothing cap -> 38V down to 31V on the first reg -> 31V to 24V on the second -> 24V to 18.6V on the SR. Both the first regs require a reasonable heatsink, 4 Cdeg/W, which gets only warm. The 17 Cdeg/W heatsinks I had on the regs originally became so hot as I couldn't hold them for more than about 10 seconds! Removing the phono card didn't make any real temp difference. I did measure the current once and I can't remember me concerned that it was too high. in the 150-200mA range. At the time I thought it strange, all this heat for little current. Could it have been an oscilation(?) somewhere?

In fact when I first built the DIYCap I remember coming down stairs one morning to hear the speakers ticking at a regular beat, about 1Hz, certainly not mains hum. My investigatation lead to me putting a heatsink on the two regs powering the phono supply rail. This cured the problem and I gained a sore finger when I checked the temp of the regs.

Also, I don't remember the noise when I was using the DIYCap on its own, I know Naims do hiss anyhow.

So things to do: Pack, no, no! Wire the DIYCap back on to the pre's power rails and try the SR's in the DIYCap (if they'll take 2V over)

One item of note: Andy, when I was testing the pre-regulator function, as in Section 8.1.3 of your excellent SR documentation, I had a reading of 3.1V rather than the 2.5V indicated in this section. I then built the second SR without referring back to the cct of the first, just incase I had made a mistake. Testing the second one, as 8.1.3, I again got 3.1V. Very strange.

And I should mention, I hate hifi paranoia, that there is something weird in the bass department too. Whilst the SRs have brought such great depth and control to the bass they seem to have brought infra-bass too. Told you it is starting to become paranoia :) Every so often I feel bass so low (not in time to the music and I didn't think my humble ES14s don't go that low) that I can't pin point any direction of source, but it is as if someone has banged on the other side of the wall, a dull thud. And before you ask it's not the neigbours! I am starting to believe that it's a room problem, only 11'x12' though, strange.

That's enough of my ramblings...

Thanks for the suggestions.
 
Why not feed the SRs with the 31V from the first regulator stage and have them output 24V to drive the amp......
 
Garf,

I'm not so sure that you're experiencing the same problem as I did (flat sound), In your case, it seems you've got a more fundamental noise problem, which I think needs fixing first. Could well be the same root cause though.
Obvious question but have you checked your SR wiring and component values. Dont know the circuit details but if Andy says you should get 2.5V across the pre-regulator, I'm sure you should! The fact that you get 3.1V indicates that perhaps you've either wired it wrongly or have a wrong value (resistor?) somewhere! I'm sure Andy will advise.

There's a few others here running reg's internally in 72's etc, and a colleague of mine has something like 8 in his 102, all running at similar voltages to your and that's the best preamp I've ever heard
Click to expand...
Andy,

My technical brain tells me that my NAC92 should also run happily at 20V but my ears told me otherwise, so I am puzzled. It's possible there's something wrong with the way I've implemented my internal regs? I was convinced at the time that I had done all the right things but as the sound was so wonderful at the higher voltage, I didnt dwell on the problem.

I'm intrigued by your colleague's 102. Has he used normal 3 terminal regs or your super regs? Also what is he using the 8 rails for. Does the 102 have a different circuit topology to the 32, 72, etc? which requires more rails or has he split the supplies to the "standard" buffer and gain stages even further?

Similar question on your 112, which runs off 15V. Does this have the same/similar buffer circuit to the early series of preamps?



John








.
 
i might be able to help on the 102.

In addition to the NAPSC supply for the switching control, i reckon it needs 9 power lines:

input buffer x2

time aligned filters x2

gain stage x2

tape buffers x2

other control (front panel light?) x1

These can be identified by locating the 27 Ohm resistors on the power rails. You can lift one end of these to isolate the circuit stage from the normal power rails and connect in your regulator boards assuming they are internally mounted (there is plenty of space inside the box to do this).

If you don't use the tape buffers you can leave these without power, and the control/lights doesn't need a special supply. Thus you can get away with only 6 regulator circuits (buffers, gain and filters)

There was a good thread on the naim forum covering this earlier this year.


http://naim-audio.infopop.net/1/OpenTopic?q=Y&a=tpc&s=67019385&f=48019385&m=1091934006&p=1
 
Originally posted by Jo Sharp
Why not feed the SRs with the 31V from the first regulator stage and have them output 24V to drive the amp......
Click to expand...

Jo,

Good point. I will try this too. so that's three actions :)
 
Well, it worked!

Back from holiday I spent a happy afternoon wiring the SRs (now running at 25v) in my DIYCap, and everything is right with the world. One happy bunny.

Okay, so the SRs are getting a feed of 37v, they're not complaining yet...

PS. China was wonderful
 
Okay, so the SRs are getting a feed of 37v, they're not complaining yet...
Click to expand...

They should be fine, there's no input cap to worry about the voltage rating of and the rest of the circuit is working within it's limits.

Make sure there's some heatsinking on the pre-reg though, at least.

Andy.
 
Originally posted by chiily

PPS. I'm running a 4 pin black SNAIC directly from the pre to the power and a 5 pin black SNAIC from the DIYCap to the pre to separate the power and signal.
Click to expand...
Just a thought, and I'm sure you will have done this, but did you disconnect the 24V feed from the power amp which would be carried over the 4 pin snaic to the pre?

Ashley
 
Ashley,

I've removed the pre-amp PSU from the power amp a while ago. Snipped the AC leads and place heatsrink over them. The PSU cct sort of went pop when I foolishly grounded the 317 case to the amp case.

The real reason from running with both the 4 and 5 pin SNAICs running in to the pre is that I had to remove one of the signal lugs on the 5 pin DIN mounted on the back of my DIYCap, due to the star earth being a large collection of wires and touching this pin. Aren't DIN sockets a pain to solder!
 
Originally posted by Andrew L Weekes
They should be fine, there's no input cap to worry about the voltage rating of and the rest of the circuit is working within it's limits.

Make sure there's some heatsinking on the pre-reg though, at least.

Andy.
Click to expand...

I've mounted each SR on to the largest of these
 
Are well you see, 'cos I was having heat problems with my diy regs I bought these to sink the finger burning heat that two of the regs (pre and main reg) were producing. Yes the heatsink did get warm, well middling warm to hot. The SRs barely warm them.

I just can't help over engineering stuff...book cases, cabinets. Am I the only person who screws and glues IKEA flat packs together?
 
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