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Topics - Alexius II

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Schematics and Layouts / Strange TDA2030 amp
« on: March 12, 2012, 04:26:20 PM »

I've drawn this schematic from a pcb of the amp I'm trying to rebuild... but it seems very strange to me.
I've double checked everything, but I can't seem to understand how it works.

Do any of you see a mistake or something? xP

Amplifier Discussion / My new amp (solid state "mesa recto")
« on: July 29, 2011, 03:41:07 PM »

I've been building a small amp for the past week... and now it is nearly finished (finaly)  :)
Here are some photos and comments ;)

First I drilled the alu chasis (hammond 1444) according to my plan:

Then I designed the power supply and the jfet splitter (which I later ditched):

Assembled power supply:

(18V for the amp and 15V for the preamp, both regulated)

I then fiddled with the global layout and stuff:

Next was choosing a handle and wood type... and then drawing a plan in 3D (google sketchup):

This was the internal pcb layout...

...until I wired it and plugged it in  :'(
There was just no way in hell my dr.Boogey would cooperate with my small dpdt switch. I used shielded&grounded wire, but the proximity of I/O wires at the switch caused it to oscilate. I then ditched the second preamp and wired the Boogey differently. It is now a "one trick pony" amp, but it sounds really good  8)
Also, I wired the volume from tonestack directly to amp input. I had some trouble with the splitter, I might try it again later.

This is from today's testing:

After three hours the heatsinks (scavenged from a dead PC PS) were just slightly warm. Transformer was a bit warmer, but still at lower than body temperature... yay  :tu:

Now I'll have to wait until monday for my wood to continue :)

Comments are as always most welcome!

Amplifier Discussion / Valvestate 8020 poweramp (LM1875)
« on: June 07, 2011, 05:13:39 PM »
Hi, I need some help/advice.
I am building myself a practice amp. I already have the preamp, but I have yet to build the power amp. I wanted to make something based on LM1875, because I already have the chip and also a 2x15V 30VA toroid transformer (which according to datasheet should give me 15W).

While googling for help about the "mixed mode" feedback configuration I found out that the Marshall Valvestate 8020 features a power amp that seems to be exactly what I was looking for. I decided to borrow that design  8|

Here is the schematic:

What I need the help with is the removal of headphone and line out (D.I.) connections.

Here is my redrawn schematic:

I marked three things I believe I understand: the zobel network, the line out and the remaining headphone out resistors.

I then removed the line out and headphone stuff and got this:
(zobel is drawn very small, so that the main topology is more obvious)

So, I just want to be sure... will this work, or am I doing something wrong?
I've never built anything like it, this mixed mode stuff is all new for me :tu:

EDIT: also, what is the meaning of that 47k and 10uF? I don't see that on the ESP project  ???

Preamps and Effects / Converting jFet overdrive circuit from 9 to 18V
« on: March 21, 2011, 09:41:08 AM »

I'm building a small amp (head) powered by LM3886. I will be using "" PCBs for the power amp and power supply. However, for the preamp I want to use a "marshall flavoured" circuit from a stompbox which I really like: Thor (from I've built a few of them, and they are great (at 9V that is). Between the preamp and power amp I would like to have a full (bass/middle/treble) tone stack.

I would really like to run the preamp at 18V to get more voltage gain out of it - so that the tone stack loss wouldn't be such a problem... and this would hopefully get me a bit further from the noise floor. :) The 18V would be regulated from about 25V (the amps power supply). The preamp schematic is here:

I do understand most of this circuit, but I still need a bit of help, if anyone would be so kind.

The first stage is a common source amplifier (looks like a fetzer valve). Second stage is again a common source amp. With double voltage, I would have to re-set both of their drain resistors/trimpots. If that much gain would result in too much distortion, I would use lower gain jFets and source/drain resistors would be replaced accordingly.

So far so good... but now comes the part I don't understand: the Mu amp  ???
I've searched the internet and couldn't find any understandable (for me) explanation on how is this thing biased and how are some values chosen (for example 390r on source). Is this thing supposed to work with higher voltages without modifications, or do i have to change anything? I suppose higher voltage would give me greater headroom?

After that you can see an opamp biased at half the supply. I doubt this part of the circuit needs any changes. Maybe I could use the other half of the opamp as an buffer, to help drive the tone stach that will follow?

Any help would be much appreciated! :tu:

Preamps and Effects / Need help with ROG: fetzer valve (gain stage) + buffer
« on: September 22, 2010, 11:35:34 AM »
Hello everyone, my first post here on SS guitar!  :tu:

I would like some opinions about a thing I'm trying to do.

I have a big tube amp (rack), but it's quite bulky.
Using it every time I have an idea and would like to record it... well, too unpractical :-\
(takes too much time to plug everything, to warm up tubes, to set the microphone etc.)

I decided that for these quick ideas a "direct" recording would be the way to go (guitar into pc + some amp simulation software)
Since my soundcard does not have a high impedance input, I have to make one :)
(input on soundcard is unbalanced line-level)

So, here is the idea: I would like a high impedance input + some voltage gain. Since I like the sound of the "Fetzer Valve", this part is covered. I would also like a low impedance output, so I need a buffer.

I did these schematics in DIY Layout Creator, so you can see exactly what I'm doing:

Then, my idea was to simply join them together like this:

And now: the questions:
1. Would this work? (why / why not)
2. How can I improve this circuit? I've got Fetzer valve figured out, but I have no experience/understanding of buffers.
3. Do I really need the in/out capacitors in the buffer? How to they work (what do they do)?
4. Do I really need the 1M gate-source resistor in the buffer? Does it add any noise?
5. I like the 2N5457 in the fetzer valve, but would the MPF102 be better for the buffer?

Looking forward for your help... and thank you in advance :tu:

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