However, if bass boost is still dear to your heart, here is probably a better way to do it. Typical opamp feedback loop compensation. This scheme doesn't load the tube as heavily.

Stoopid with Opamp Bass Boost

[Edited by runeight on 10-30-2005 at 06:13 PM.]

Thanks for the replies mains and batman and runeight, I'm still a bit new in all of this. Could you please explain the relationship between the freq and the cap value? Is this the same as the freq shelving formulas used by for the PPA and M3 amps?

I've already drawn up the layout for the first bass-boost design, I'll modify it later tonight to use the new schem.

Thanks again guys.

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quote:

[By runeight] Mains and Batman are no doubt correct about the bass boost. They both did a fair amount of component adjusting to fine tune the response of the amp.

However, if bass boost is still dear to your heart, here is probably a better way to do it. Typical opamp feedback loop compensation. This scheme doesn't load the tube as heavily.

Stoopid with Opamp Bass Boost

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quote:

[By ruanjun] i'd like to put 10k volume pot instead of 100k.. do i need to change anything?

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No problem if your sources don't mind the lower input impedance.

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quote:

[By pho_boi] Could you please explain the relationship between the freq and the cap value?

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Using the values for the cap and resistor that are in series in the feedback looop:

f = 1 / (2 * pi * R * C)

where R and C are in ohms and farads. In the drawing that I've shown:

f = 1 / (6.28 * 1e6 * 470e-12) =~ 339Hz

But, I'm kind of wishing I hadn't posted the bass boost version since mains and batman are certain that the amp doesn't need bass boost and because leaving out the boost makes the amp simpler.

[Edited by runeight on 10-31-2005 at 11:02 AM.]

I currently don't have a 15-0-15 transformer in my parts bin. But I do have two identical 0-15 transformers that I can use. Would it be possible to wire up the two 0v terminals together and tie it to GND, so that it works like a centre-tap transformer?

Thanks.

What you really want is 0-15:0-15 so that the outside 0 and outside 15 are out of phase.

What is the current rating of the trafos?

[Edited by runeight on 11-01-2005 at 07:37 AM.]

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quote:

[By runeight] Yes, but you have to get the polarity of the trafos correct.

What you really want is 0-15:0-15 so that the outside 0 and outside 15 are out of phase.

What is the current rating of the trafos?

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The transformers are rated at 30VA, so I think that means 2A.

I'm not quite sure what you mean, but do you mean that both transformers have to be wired oposite to the 240v AC from the wall? so that the output voltages are out of phase?

ahaha sorry if I'm not making sense

btw this is the transformer that I have:

link

[Edited by pho_boi on 11-01-2005 at 07:48 AM.]

Given that these are 2A transformers, it might be better to use the 12.6V secondaries. Can you measure the AC voltage with no load for both the 15 and 12.6 secondaries?

Your wiring diagrams may show the trafo like this. If so, wire it this way. The 0:15 connection in the middle is your center tap and is grounded in the Stoopid power supply. The 0 and 15 connections at the top and bottom go to the bridge.

[Edited by runeight on 11-01-2005 at 08:27 AM.]

True, these transformers are larger than really needed, but at 8 Oz $ each, a pair doesn't bust the bank.

I'm begining to think us UK folk are lucky with whats available for small PCB traffos .. which is a surprise as I've always thought the USA would have a far better choice than the rest of the world ????
But this doesn't seem so, and Oz seems lacking despite being so close to where the darn things are made.
I'm confused.

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quote:

[By mains_hum] Yes, you can use two transformers.
If the datasheet doesn't make it clear as to polarity (phase) ... and I'm assuming you have two transformers that are the same part number, if you lay them side by side then runeight's diagrams are just as you see the tags in real life, so this should make it easy to get the wiring correct.

True, these transformers are larger than really needed, but at 8 Oz $ each, a pair doesn't bust the bank.

I'm begining to think us UK folk are lucky with whats available for small PCB traffos .. which is a surprise as I've always thought the USA would have a far better choice than the rest of the world ????
But this doesn't seem so, and Oz seems lacking despite being so close to where the darn things are made.
I'm confused.

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Yeh, I find that Australia is really lack in things like these. And if you do find the parts you want they usuaully cost a fortune compared to overseas. ATM I am having trouble finding a good quality hole-matrix prototype board to build my amp on. The ones found at the shops around here are really bad quality and sometime the pads around the holes just fall off .

The original Stoopid is designed with a 200mA trafo so that when the tube heater is attached it will pull down into a suitable voltage range.

Can you measure the no-load voltage for us?

This one looks like a likely candidate for people in the US:

http://www.alli...=&DESC=AHF06030

Disclaimer: I've never done business with the vendor and don't know if they have a minimum order amount or what.

Edit: For three times the price, from mouser:

http://www.mouse...e_pcodeid=55302

[Edited by ericj on 11-01-2005 at 06:13 PM.]

30VCT/400mA might be close enough for the voltages to be ok depending on its regulation. A little extra voltage from the trafo will increase the B+ for the tube. This is a good thing to have happen. OTOH, it also increases the input voltage to the bipolar regulators and may cause overvoltage in the PS caps.

The input to the regs must be kept below 40VDC. The caps can be adjusted for any reasonable trafo by making all the 63V caps 100V caps.

Part of our purpose with the design and component specs is to keep everything as small as possible and as cheap as possible. The spec on everything with a 30VCT/200mA trafo gives enough room without over-specifying.

[Edited by runeight on 11-01-2005 at 06:30 PM.]

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quote:

[By runeight] pho_boi, if you're going to use these transformers, it is still important to know what their voltage is with no load. I would guess that they have about 25% regulation, in which case using the 15V winding might cause some overvoltages in the PS.

The original Stoopid is designed with a 200mA trafo so that when the tube heater is attached it will pull down into a suitable voltage range.

Can you measure the no-load voltage for us?

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Sure runeight, I will have to check them later tonight though. It's exam time at University and I shouldn't even be on the forums!!! But I will take a break a little break at night and measure the voltage from the 15 and the 12.6

Allied is reliable but they have a $25 minimum order

But, I don't want to interfere with your exams.

[Edited by runeight on 11-01-2005 at 08:59 PM.]

Just did the measurements of the transformer, and I got 16v from the 15v and 12.7v from the 12.6v

Yep sure, I'll build the Cavalli's Stoopid Dream 1.03 using the 2k trimpot instead of the 2k5. Someone will probably build one before me though, I should really be studying my Cell Molecular Biology

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quote:

[By runeight] Say, I hate to bug you on this, but the numbers show amazingly good regulation on those transformers. Either that or under load they will dip below their rated voltages. May I ask how you measured these? Very interesting.

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I measured it using the AC function on my multimeter straigh from the 15v and 12.6v lugs on the transformer, so there was no load on them at all.

[edit] forgot to mention that the red lines are jumpers that are ontop of the board.

[edit2] i forgot to mark the orientation of the diodes, but thats pretty straight forward]

schem

layout

[Edited by pho_boi on 11-03-2005 at 03:39 AM.]