Minicap6 based integrated amp PSU

[mikesnowdon]

Hi

If building an integrated amp based on ZAP250 poweramps and dual mono class-A discrete preamp's with on board low noise regulators - with limited space - what would be the best way to tap into the minicap6 to power the preamp?

Ive been musing over this for a while and I have a plan which Im fairly confident about but I just wanted to know others opinions on the scheme and possible ways it can be improved.

The whole reason for tapping into the poweramp supply is one of limited space, and simplicity. The preamps (SKAaudio 'SKpre') have the facility for a on-board diode bridge/smoothing but space being tight I have opted for the above solution (no room for caps). I could squeeze in a 7VA trafo but that would leave me needing to squash in a separate bridge/smoothing section somewhere else.

So you can see that tapping the minicap's (via dropper resistors and 24v zeners) is a nice simple soloution. It also means the L/R preamp modules have dual mono power for an added bonus.

Here's what I have planned:

http://img81.imageshack.us/img81/228...edamppsuv3.png

Obviously zeners are noisy, however Greg assures me that the Skpre on-board regs will keep this from affecting the preamps. But what about the zeners affecting the poweramp supply?

This purpose of this post is basically to be a final check before I start wiring in the preamps, to make sure its as good as it can be within the limitations of the build. any suggestions to improve on this are very welcome

TIA

[mikesnowdon]

This is the SKpre on board discrete regulator stage:

http://img200.imageshack.us/img200/4...eregulator.png

[bivalve]

Mike,
I don't think the wiring of the bridge is quite right. Wiki 'Diode Bridge' doesn't have exactly what you want but shows separation of input and output.

[mikesnowdon]

Its just for show. Its just the first thing I picked in the component library (using ExpressPCB). Purely for illustrative purposes only.

[mikesnowdon]

Why no replies?

Is it because Im bucking the trend by building and 'intergrated'? Or because it cant be improved? Or maybe its all good and nobody has any ideas?

...why no replies....

[PigletsDad]

I think you would be better of using a LM317/LM337 pre-regulator to drop down from the 46V to say +/-24V input into the SK regulator; this will be quieter, have proper protection arrangements, and give a lower output impedance.

The zeners would work, but you will need high power parts.

The SK regulator is a weird circuit; the tolerance on the output voltage must be enormous, although it should be pretty quiet.

[mikesnowdon]

Hi PD.

I have a small PCB with 317/337 on board regs plus rectifier/smoothing which I could use. The 317/337's are configured as per the "Adjustable Regulator with Improved
Ripple Rejection" schematic on page 7 of the datasheet. Diodes are MUR860's with 4'700uF smoothing caps. Ive built the reg based on nationals suggested cap values/types (tants/films) and set the output at +/-22v.

Can the 317/337 take +/- 46v? Also I only have one of these regulator PCB's so whats the best way to tap into the dual minicap6 supplies?

The SK regulator is a weird circuit; the tolerance on the output voltage must be enormous, although it should be pretty quiet.

Ive thought about bypassing it using TeddyRegs but that would take some serious head-scratching to find space to mount them

[colasblue]

317/337's max out at about 37V so if you try to regulate 46V down to 24V with one there will be some blue smoke !

[James Evans]

37v is the max difference between input and output I believe. I'm sure Martin Clark mentioned on a recent thread (317/337 TPR maybe) he's running some at 50v+ input.

[mikesnowdon]

Thats what I was thinking...

Maybe PD was suggesting using zeners to get the voltage down to something that the 317/337 would be happy with? Is there a higher rated equivalent to the 317/337?

By the way Ive already tested the arrangement in the first post and it works. I dont know how reliable it is though.

and just to show you the layout:



Ive got a bit of space down the left side to play with and a little area in the bottom right corner near the AC inlet. The space on the left imediately behind he RCA's will have a dantimax input relay selector mounted vertically.

[mikesnowdon]

Originally Posted by James Evans

37v is the max difference between input and output I believe. I'm sure Martin Clark mentioned on a recent thread (317/337 TPR maybe) he's running some at 50v+ input.

Missed your post James. I would assume the regs would need small heatsinks running on this voltage?

Can anyone confirm they are ok on 50V+? Ive read and re-read the datasheet and I cant see a max input voltage stated anywhere.

[PigletsDad]

They can be used at input voltages of hundreds of volts; even thousands, because they don't have a connection to earth - they just see the resistors that set the output, and the reference pin sits a constant difference from the output pin.

The only requirement is that the difference between input and output is less than 37V.

46V- 24V = 22V, so that is OK.

The potential trouble occurs during startup, when the voltage on the output is 0, while the input is rising rapidly - you have to be sure that input voltage doesn't rise so fast that capacitors on the output don't have time to charge.

Now a 317 can briefly put out about 1A. If the output bypass is say 100uF, this will charge at 10,000V per second, so will get to 24V in 2.4ms, which is much shorter than half a mains cycle, so I think you are fine, provided the load capacitances are not too big.

[mikesnowdon]

The potential trouble occurs during startup, when the voltage on the output is 0, while the input is rising rapidly - you have to be sure that input voltage doesn't rise so fast that capacitors on the output don't have time to charge.

Now a 317 can briefly put out about 1A. If the output bypass is say 100uF, this will charge at 10,000V per second, so will get to 24V in 2.4ms, which is much shorter than half a mains cycle, so I think you are fine, provided the load capacitances are not too big.

That's good news then. Im allready using 100uf nichicon MUSE on the output of the 317/337 regs.

Whats the ideal way to tap into the dual mono power supplies then? I would imagine taking a +/-VDC feed of one side would be easiest.

Out of interest could this be done with 4 teddy regs? I have a set of TR boards ready and waiting. Maybe a dual mono +/- supply with 4 TR's could be done? The current draw of the SKpre's is low enough if the TR can take +/- 46VDC though?.

[trancera]

Why not just a normal Teddy 317 with large drop into fretlington? and bypass the SKpre regulator?

Either way, gonna be a cracking integrated.

[mikesnowdon]

Originally Posted by trancera

Why not just a normal Teddy 317 with large drop into fretlington? and bypass the SKpre regulator?

Either way, gonna be a cracking integrated.

Thanks mate.

Im going with the TR's I think. Ive been reading up a bit and the first stage of the TR is a 317/337 so +/-46VDC input is fine. Like you say a large drop - say to about 33/30v - into the gyrator section.

I might try bypassing the Skpre reg's too. Good idea. If I did I might need help working out how to s[read the dropout over the TR. SKpre's require regualted +/-15V.

Ive been looking up the parts @ Farnell (with very slow internet connection), so far Ive found the tants and 3.3uF film (Farnell dont do MKS2-XL). Cant find suitable NP0/Z7R ceramic's, only SMD versions. I wonder if Mica would be ok here? Also its surprising how much the tants cost, Im selecting them based on Teddy recommendations from his site:

#

Component choice

The choice of component is very important, especially in the gyrator part.

# The Tantalum capacitor should be of high quality, I recommend using brands like Kemet, AVX, Panasonic, Vishay, etc. Low quality Tantalum capacitors tend to leak, causing performance degradation or even changes in output voltage. Mil-spec axial Tantalum capacitors perform very well in this position.

# The Ceramic capacitor should be X7R or NPO/COg. NPO/COg are better but more difficult to source, especially in these values, SMT versions are more common. In any case X7R are good enough.

# The capacitor at the output of the gyrator has an effect on the tonal balance and can be used for fine tuning according to your preference and the rest of your system. 10-20uF Tantalum will give a warm sound, a 1-10uF film is a good all rounder. A 0.01-0.1 will give very transparent and accurate sound. Combinations work as well 10uF Tantalum bypassed with 0.01-0.1uF film gives the best of both worlds.

# The D44H11 is the best choice I've found for the output transistor. It has good power handling and works well at high frequencies. For low current and low dropout applications smaller transistors such as the ZTX651 (or ZTX751 for the negative version) can be used.

# For the JFET, transistors it is important to

Havent got around to the transistors yet. Hopefully I can get them from Farnell too.

PS: I worked out where to put the TR's. And I have a convenient space for a small (ca. 7VA) trafo but not sure where to put smoothing caps/bridge. I actually think I'm better off tapping into the minicaps anyway for a dual mono preamp supply.