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Messages - sim0n

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Preamps and Effects / Re: Lowering noise in bipolar powered FET preamp
« on: January 02, 2014, 06:39:00 PM »
The Randall had a clean channel too, without any diodes  :P

I've since managed to fry the power amp chip by accidentally shorting something with a multimeter probe while I trying to measure something so this project is paused until another TDA2050 arrives

Preamps and Effects / Re: Lowering noise in bipolar powered FET preamp
« on: December 23, 2013, 06:40:06 PM »
Alright, cleaned enough bench space to hook things up to take some measurements. Here is the schematic of what I've actually built, with actual component values and measured DC voltages. I tweaked a couple of component values while I had it built on the breadboard, so I attributed the noise at that stage to the fact it was built on a breadboard.

Hooked up to my oscilloscope, with a 1khz test signal from a little test oscillator, the first stage has a gain of about 9x. My oscillator doesn't have enough output to drive it into clipping but at 14v peak to peak theres some onset of distortion (I moved the source capacitor to the second stage because my guitar's pickups have enough output to clip this stage  :cheesy: )
The second stage has a gain of 37X and goes finishes in all out asymmetric relatively soft clipping with 20 volts peak to peak.

Noise-wise theres about 1mV on the power rails (and it seems to be evenly distributed over the spectrum). On the gate of the first stage with the input grounded about 1mV, on the first stage's drain maybe 2 or 3mV (measured as peak to peak voltage). With the gain set to 0% theres maybe 1.5mV on the second stage's gate and 15mV of the drain. With the gain at 100% theres 50mV p-p of noise at that point and it follows through in the rest of the circuit.

So like I mentioned, the preamp functions and sounds fine.  And with both volume and gain at 100% there is way too much output on tap as the power amp chip already clipping with and input 2v peak to peak and even after the tonestack losses theres still way too much. But I still feel the total amount noise should be lower (50mV of noise x 40x gain of the power amp = 2V on the speaker  :P)

For now I'm suspecting somethings going on with the gate voltages. As if they're not at the potential of the lower rail it would imply that theres current flowing into the gate, which can't be right for FETs  :o

edit: hmm, or my multimeter's internal resistance is pulling the gates up because I have it referenced to 0V and not -12V

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?

Preamps and Effects / Re: SMF-1: analog dry for digital boxes.
« on: December 20, 2012, 10:29:01 AM »
The reg will run very hot if it has to drop a lot of voltage.
You may need some series resistance before the reg.

zener diodes also work well

Amplifier Discussion / Re: TDA7850?
« on: December 18, 2012, 03:34:19 AM »
The TDA7850's individual channels are already bridged internally so you can't do much there and running them in parallel doesn't help much when its already good at driving low impedances. That data sheet says it rated at 50W @ 14.4V for 2ohm loads, at 4 ohms it's more in the area under 25W. You could leave unneeded channels disconnected as you proposed, maybe run a stereo setup if you have two speakers.

Tubes and Hybrids / Re: Small Push-Pull Amplifier
« on: November 27, 2012, 07:50:48 AM »
I was referring to this plug here where you can easily flip it reversing neutral/live (with ground is present at all times). Since the design itself allows this I'm assuming the people that drafted the spec allowed for it, with the third prong convering the safety portion.

Tubes and Hybrids / Re: Small Push-Pull Amplifier
« on: November 27, 2012, 02:50:28 AM »
Does it matter that much? I'm in euroland with the reversable AC plugs where you can happily switch live and neutral... Or even if you have non-symmetrical ones, the wiring in the socket itself may be hooked up the other way. And if a fault does occur between the live and ground it should be caught by the residual-current circuit breaker (but then may be plugging into 30 year old wiring without such amenities)

Amplifier Discussion / Re: Fixing this old stereo amplifier
« on: August 16, 2012, 06:53:03 AM »
Huzzah, it works! I wasted two hours yesterday wondering why it was giving me -18V on the output until I finally realised I put a PNP transistor in for Q4 instead of an NPN  :duh

I replaced all the smaller transistors with MPSA06/MPSA56, the driver transistors for BD139/BD140 and just replaced the dead NPN output transistor with another 2n3055 that I already had.

Here one channel hooked up on my temporary test jig:

You can see the age of the transformer  :-\ but it functions. The power supply only has 2x4700uF, I'll probably redo it with some bigger caps and proper fuses and such.
Now to just get an enclosure for this thing and turn it into a useable piece of equipment  :cheesy:

Thanks for the help guys!

The tremolo section shows a lightbulb and ligh dependant resistor. The most obvious to thing to check would be if the bulb has died.

As for the crisper sound: it might not be the amp but actually be the speaker and/or cabinet responsible for this. So keep that in mind before you redo the entire thing.

Amplifier Discussion / Re: Fixing this old stereo amplifier
« on: August 09, 2012, 10:48:37 AM »
Hah, finally getting some real world usage of the stuff I learned at university.

After getting a set of measurements for the transformer's parameters with the scope and a multimeter, I then remembered I had one of those $15 wall socket energy meter things :lmao: (if it wasn't summer right now I could probably take it to the lab and hook it up to a shiny $5000 power analyzer)

Running open circuit its has 15W of losses in the core (and a power factor of 0.3). My own measurements gave a result of 20W. Judging by its size (the core is 10.5x10.5x5x5cm) it seems to be somewhere around the 200VA range and according to the tables in my EE handbook, 20W is nothing odd for the core losses for a transfomer of size.

edit: and yeah, I'm puzzled about the zener too buts the way its installed on the circuit and the voltage seems to be right too. It just might be a case of strange component markings.

edit 2: I've desoldered all the dead components and checked their health out of circuit

Q4 and all the silicon ahead of it is dead (short between two terminals with various ailments to the third one) except the BDX18 (the PNP output transistor) and the BD137 (the NPN side driver transistor). D4 was also shorted out and the 0.39ohm resistor on the PNP side has turned into 50kiloohms.

I soldered a couple of things in, but just realised that the largest voltage they'll be seeing is 60V, so if the maximum collector-emitter voltage is 50V that might not be good and I can't find anything in my drawers rated for higher than that thats not a full out power transistor.

edit 3: my logic has lead me to the conclusion, that the burnt out resistor probably saved the PNP output transistor from death

Amplifier Discussion / Re: Fixing this old stereo amplifier
« on: August 09, 2012, 04:19:18 AM »
If its supposed to adjust the offset to zero then it should do that!!  :duh
This falls in the category "I dont know exactly what I'm doing and how it works" but I changed R2 from 3k3 to 2k2 and that shifted the range of adjustment to let me got 0mV (well, I know its got something to do with Q3 being a constant current source but I've got no idea what the rest of the stuff is doing).
I hooked up my scope, everything looked fine on the output, hooked up a speaker and it works too :D so...yay! It hits 57volts peak-to-peak on the output. It also appears to be quiet apt at amplifying the MHz noise my computer puts out along with the audio signal  :-\

But I've begun to suspect the transformer is a bit iffy too, as it also looks like a homemade job (or its just that old, theres brown butchers paper between layers of windings). After an hour or so of reasonably quiet playing it got too hot to touch, way way more than the transistors. I'm suspecting that whoever was stacking this didn't bother to do any kind of isolation between laminations as the core gets hotter than the windings. In the original enclosure the fan was pointed at the transformer, not the heatsinks.

So I finally get to use all that knowledge about how to measure transformer parameters I learnt at school  :lmao:

Amplifier Discussion / Re: Fixing this old stereo amplifier
« on: August 08, 2012, 01:25:25 PM »
I'm beginning to wonder what you need us for, since you seem to be getting on just fine on your own (heh heh   ;) ).


I checked the heatsinked parts for continuity, its not visible on the photo but apart from the ugly wiring, the output and driver transistors are actually properly isolated from the heatsink with a mica pad and those plastic washer/spacer things.

I finally rigged up a test jig and got everything connected together. The secondary of the transformer measures 22-0-22. The power supply unloaded measures +-30 volts.
I hooked up the working channel with the lamp limiter and the input grounded. The supply voltage drops to +25. I managed to measure a set of votlages and they look OK for the most part, or at least they roughly match up to the spice model.

The 2k2 trimmer for the bias current functions, the sweep is a bit finnicky but if I turn it too far the lamp noticeably lights up so it seems that that works (and I managed to measure the voltage drop over the 0.39ohm resistors and set it precisely). However, the 10k trimmer, will not adjust the dc offset into the negative range, it doesn't go below 30mV at all.
Any ideas for that issue? My intuition says to check electrolytic capacitors (specifically C2 which is a 10uF bipolar).

Amplifier Discussion / Re: Fixing this old stereo amplifier
« on: August 07, 2012, 11:15:33 AM »
Thanks for the replies :)

Here is the transistor mounted on the heatsink (the TO3 package is visible on the other side, the 150ohm resistor soldered directly between the emitter and base with the driver transistor screwed on nearby). This is NPN side, the other side is identical, the circuitboard is sandwiched between the two heatsinks with standoffs.

Among other visible casualites, along with the Q11 and R20, I also noticed that Q12 has disintegrated (again, one pin completely disconnected from the cracked pacakge) and Q5 is chipped away on the side as well if the sheds any light on the subject. The junction between the base and collector seems to be shorted.
Among resistors that have iffy values are R13 and R19 (around 200ohms instead of 1k), R10 (700ohms inteas of 3k3), R11 (also around 700 instead of 1k). I'm getting conductivity over C6 too. I wrongly read the colour bands, R16 and R20 are actually 180ohms, the burnt R20 measures 140ohms.
And if I measure from the positive to the negative power rail it shows 370ohms.
On the other channel of the amp all these components measure what they visually indicate with any abnormalities. So it looks like the output transistors are shot too.

I'm still in the process of building the lamp limiter and rebuilding the power supply (the bridge rectifier measures OK with my multimeters continuity test). I'll try and get the other channel working before I tackle this.

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).

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