Questions regarding Pedra Rogics BIGC

[justblair]

Hi All

I am designing what will be a very compact (220mm*100mm*55mm) amplifier for my GF. For aesthetic reasons, I am limited to a single box solution. This gives me issues to resolve.

Question 1.

I am limited to a 120vac transformer. I intend to use a 18-0,18-0 tranny because it fits, I have 25v panny HFQ caps that I want to use (size wise they are just right) and then regulate to 18v DC using the regulator

Regulated supply

This should be by my cigarette packet calculations be suitable for a low powered stereo amplfier driving into 6-8 ohm speakers?

Am I right?

Question 2. Am I right in saying working with lower voltages will generate less heat in the system?

Question 3.

The circuit for the Rogic buffer includes a 4.7uf Cap between the buffer and the amp stage. In my own setup, I gained a big advantage from omiting the cap, the amp is supplied by a kookaburra which has no DC blocking after the IC opamps. Does the Jfet buffer itself create DC offset, or does the cap protect from offset from the source?

If I have to use a cap, good quality poly caps (such as sonicap or mundorf) are my preference, but they are large when you get to 4.7uf values and this makes my design more dificult and resultant pcb's very tightly packed.

The buffer

I intend using lm7815/7915 regs followed with 47uf Rubycon ZA caps to supply the buffer.

[I_Forgot]

The 7815 and 7915 are not approved for audiophile use.

You gotta make up some Jung regulators if you want to impress anyone. If that's too much trouble, use Linear Technology three terminal regulators. Not equal to Jungs, but merely sniffed at by audiophiles compared to the bashing you'll have to take if you use the 7X15 parts.

Why would you use a regulated supply for the amp? If a transient comes along that exceeds the regulator's current limit, the reg will shut off and the waveform will clip. Without the regulator, the the amp will just keep sucking on the caps and clip more gracefully. Regulators drop a few volts. Those volts would be better off delivered to your speaker as audio power than as heat dissipated in a regulator.

I_F

[justblair]

I know the 7815 and 7915 are not the best. I was looking for something and Pedja mentioned on his site he had used them. They are cheap, and more importantly dont require much in supporting components.

I was trying all weekend to come up with a single board design that would fit both the power supplies, amps and buffers on a single board.... Just about possible, to fit all the components on. However I am sure that the compromise would be in the listening. Tonight I will try the same with stacked boards.

What LT regs would you suggest? I dont think I can free up enough real estate for a super regulator.

The point in using a regulated supply is, according to those that advocate it, the regulated supply has the advantage of producing a very stable voltage, but retaining low capacitance at the chip. I have some sikorel 125 470ohm caps to try out at the chips pins. The reg'd supply will hopefully let me use them to good effect.

As for clipping, The qualty is more important than power. The speakers are currently mission 760i (Oh my goodness these are great little bookshelfs. They sat in a cupboard for years, only mods are a rewire but they sound great ) These are pretty efficient, and I will be going for efficient speakers when it comes time to buld the upgrades (metronome probablly)

Of course I can only try out the regulated supply and see how it sounds, but loads or people have used regulated supplies and give favourable comments.

[Nordic]

As long as you have some sort of relay protection, you should be fine... I use just the buffer part, and I know it has quite a power on off thump without shorting its outputs with a relay. I used cheapass poly caps, from the hardware shop... those standard Jamicon sqaure types ... sounds awesome.

[Dr.EM]

The LM317 and LM337 regs are quieter and you could even add something like this after to clean up the supply further:

http://www.techlib.com/electronics/finesse.html

Can't see a reason to use anything "better" really, this offers an incredibly low noise and well regulated supply.

[justblair]

Hi All. I have another question.

I have been looking at the issue of volume control. In the arrangement I am interested in. Using a sereo pot at the front of the case makes little sense. The torroid wil be at the front, and I dont want to run a signal past it in close proximity.

So one candidate so far is using a MAx5440 volume pot with a rotary encoder attached. This is very attractive to me, because it will give an led readout of volume, has mute features etc

The wipers from the chip are buffered.

If I go down this design route, should I just forget about using the jfet buffer?

Another way of putting this, is there still an advantage in using the Jfet buffer even though the volume control is buffered itself?

Regards

Blair