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Topics - sim0n

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Preamps and Effects / Lowering noise in bipolar powered FET preamp
« on: December 12, 2013, 09:29:05 AM »
I've built myself the green channel (clean) of the famed Randall RG preamp to along with my TDA2050 power amp. It more or less the same thing as the famous red channel but with a treble bleed capacitor on the gain pot and no clipping diodes. I did however make some adjustments based what components I had and the amp it was going into:
  • moved the master volume to after the tone stack
  • made the entire thing run off bipolar 12V power (so that slightly changed the layout of the biasing)
My schematic is in the attachment. Part values are as the original's (heres a redrawn schematic of the dirty channel ),  33kohms on the drain, 2.2kohms on the source, I used 4.7kohms on source of both of the buffer FETs. A 100k gain control and a 100k volume control. My power supply is a 7812 and a 7912 linear regulator.

Alltogether it sounds pretty nice, a nice JFET overdrivey sound. The issue I have is that I suspect its more noisy than it should be, possibly due to the changes I made in regard to what reference voltages are.

With the input grounded, If I set the volume to 0 its dead silent, if I put my ear up to the speaker I can barely hear 50hz hum.
If I set the gain to 0 and thus effectively mute the first stage, theres a waterfall of noise coming from the second gain stage alone whenever volume is past 50% (where it needs to be for full clean volume if I set the gain control to around 25% for the cleanest of cleans with my hot pickups  xP ).
If I put a mic in front of the speaker and watch the VU meter in my computer I can measure that each stage is adding about 12dB of white noise.
To be fair, this is mainly evident when dimed, and with signal applied from 2/3 master volume and up the power amp is already clipping but still, lower noise would be better tjomg.

Is this just a fact of life with preamps of such design? if I were after a quieter amp I should have used something you're actually adjusting the gain of the stage and not just attenuating the signal afterwards? Use clipping diodes instead of clipping the transistors themselves so you don't have to amplify the hell out of your signal to get clipping?

Amplifier Discussion / Fixing this old stereo amplifier
« on: August 07, 2012, 04:45:47 AM »
Hey guys, I guess I'll try this here as well cause I know there are a bunch of knowledgeable folk around. My friend gave me this amp that had been sitting under his bench in his workshop for the past 6 or 7 years. The enclsoure made of MDF was all warped and had mould growing over half of it. The amp was supposedly a DIY job he and a friend of his made (the actual circuit seems to be from some kit). According to him, one channel didn't work, the other was allegedly ok.

Anyway, the wiring was absolutely horrid, nothing in the enclosure was actually screwed down other than the transformer and the enclosure itself was probably a biohazard so I dismantled the entire thing.

I've managed to draw myself a schematic of the circuit (and it also functions in LTSpice so I guess I got it right)

As you can see on the photo above, theres some black charring on the circuitboard so I'm guessing that this is the channel thats shot. Though the resistor that has turned black measures ok (R20 in the schematic), the base connection on the transistor Q11 (as numbered in my schematic) is ripped out of the package (not visible on the photo, the driver transistors are mounted directly on the heatsink with the main output transistors).
So, how would I go about fixing that issue, since usually when one things dies it likes to take other things with it.  Is there anything else that I should check? How close does the replacement for the transistor need to be? I've got some TIP32C and a multide of common TO220 package transistors, would any of those work as a replacement or should I order something better?

And, if I do fix it, what am I aiming to get when adjusting the trimmers? (I've got a scope and a handful of multimeters for measurements).

I buggered up my amp a long time ago (Laney TF50, 30watt combo, based on a TDA2050 chip) and have finally got around to maybe making the thing play again. I ransacked the original board for parts so basically I'm rebuilding the amplifier from ground up, keeping only the transformer and speaker. I'm going again with the same chip. I found the schematic of the original amp on the internet, it has an extra section of feedback compared to the datasheet typical application circuit from the TDA2050.

Here is it redrawn (and I removed the headphone output) with the original component values

I can't really get the funciton of R6,R7,R8 and C4. Is this some kind of high frequency roll off thing to prevent oscillations? What advantage does it offer compared to leaving that section completely out?

Amplifier Discussion / going to redo my amp
« on: July 31, 2008, 06:32:21 AM »
I've got a Laney TF50, a ten year old 30 watt solid state thing.

recently the reverb died (one of the coils on it seems dead cause I got springy sounds if i hook it up in reverse...just no guitar) so thats a good enough excuse for me to mess with it (find another use for that knob!)

heres the schematic

basically I'm going to keep the poweramp as is, just rebuild it on my own board. Only going to get rid of that headphone output section. Should be ok, right?
its a bit different from the TDA2050 datasheet circuit as its got that extra feedback loop in there... but its worked so....

and for a heatsink its just had a aluminium plate connecting it to the chassis
enough? again, its worked like that till now so I figure there shouldn't be trouble....

I'm going to leave the power supply as is mostly too, just on my own board (same transformer...going to get new capacitors). Probably going to just throw on 7815/7915 regulators for the preamp supply instead of those zener diodes, shouldn't be any problems with that either?

I'm going to get rid of the preamp entirely, which is actually pretty interesting as its made out of logic inverters but its gets all mushy when you turn the gain up. Going to probably replace it with something based around JFETs but getting the poweramp right is my first priority.

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