I bought this "Starmaker 8015" combo to use the cabinet for my first diy build. I got it for 10€ and assumed it is just some asian made department store crap - although the cab looked very well made. Now I opened it up and see that there are almost only british made components inside; the jacks are original cliff, the switch is a swann 51 (jcm800 style) and the speaker a Celestion G10-20!
The pots are RadiOhm and the electrolytic caps are made in Japan.
It also sounds very nice, if you don't dial in too much bass, this is a clean only amp, there is no distortion circuit.
So has anyone ever heard about this amps? And where they were offered in the 80s? Google only shows me some classifieds where these amps were offered, but mostly in (eastern-) european countries.
Oh man, what an odyssey I've been through...
I wanted to know what can I improve on this design and ordered new caps, opamps (NE5534) and power transistors.
The amp has a sweet sound, but all of the tone controls, the more they were turned up, added some background hiss on notes, reminiscent of a noisegate. I guess that's because of the CA741 opamps. And then there was also a 100Hz buzz (230V/50Hz wall supply) which increased exponentially when volume knob was above 50%.
So my first step was to renew all electrolytic caps. Noise problems were still there, but I got a noticeable increase in maximum bass and volume.
Then I swapped the opamps from CA741 to the NE5534s. On the first test the amp was dead quiet, no signal, only the mid shift/gain circuit could self oscillate with fart-like sounds. Then I checked Vcc on the opamps (the CA741's before had 22VDC), and I only measured 3VDC!
Then I analyzed the opamp power supply, and spotted a series 2.2k resistor in front. There is 40VDC coming from the rectifier, and 37VDC were falling on that resistor, leaving 3 volts for the opamps (there are also a 24V zener diode and a 220uF cap involved).
With crocodile clamps I tried different resistors in parallel, and with finally around 800Ohms series resistance I got 22 volts at the opamps again. So I soldered a 820Ohms 2W resistor in place of the 2.2k old one.
Now the amp worked and finally lost the background hiss but I still got that 100Hz buzz at higher volume. It was also there when nothing was plugged in.
I checked all supply circuits and there was no excessive ripple or something like that.
Desperate I also changed the output transistors (one of them had burn marks anyway), but this didn't solve it.
Long story short, in the end it were the input jacks, the switching contacts to ground on both were dirty and had high resistance...
Tomorrow I can finally do a meaningful test run on this little amp.
The amp was working before with the 2K2 resistor, so changing it is just a 'band-aid' solution to some fault in the low voltage powered circuits.
You need to repair it rather than modify.
37V across 2K2 was 16mA of current, where was it going?
The amp was working with the 2k2 resistor when the CA741 opamps were in there. When I changed them to NE5534 there was more current needed.
Min-Max Currents:
CA741: 1.7 - 2.4mA
NE5534: 4 - 8mA
So current draw is approx 4 times higher with the new opamps and the series resistor was limiting the current.
Yes, I see now.
From the voltage drop (across the 2K2) you had with the 741's, assuming you are again at 22V with the 820R, you have increased from 8mA to 22mA.
So there is no defect, just an imperfect part substitution. TL071 would have been a direct drop-in replacement with no mods.
The 5534 often needs a small compensation capacitor between pins 5 and 8, usually 22pF.
Without that it might be oscillating thus drawing more current.
Ok, thanks. Would I also hear that oscillations if they happen?
I hear some background fuzz underlying the notes.
Quote from: Maddus on January 12, 2024, 04:22:45 AMOk, thanks. Would I also hear that oscillations if they happen?
I hear some background fuzz underlying the notes.
Typically opamp oscillation is above the hearing spectrum, nothing you would be able to reproduce through a speaker or hear.
What I also noticed is that I get an unpleasant clipping when the input gets too high, for example when I hit the strings a bit harder on a humbucker guitar. I tested with a sine wave generator, and the clipping begins at 0.5 volts, regardless of set amp volume. Is this normal for old solid state designs? And can I modify something to prevent input clipping?
without a schematic it's hard to know but I'd guess the preamp section is causing the harsh distortion, not so much the power stage but it is a budget amp design and high chance there was little to no R&D before it went on sale.
Phil.
I think I am going to trace it this week, just need to make photos from both sides and then lay them over each other half transparent in some imaging software.
Just did some quick measurements with an o-scope and it showed at the first opamp output that the clipping is asymmetric and in the negative half wave. At the input I had 0.7Vpp and on the output around 14Vpp when clipping starts.
Seems that also the bias voltage isn't exactly Vs/2, I have Vs=22.2VDC but 10.4VDC at the input. The single supply bias resistors are 33kOhms.
If it is running the chips with single supply, it will have blocking caps at input and output of IC. Check that they are not leaky.
Also check for protection diodes at IC input.
Tracing is in progress, see attached pdf. Took me 3 evenings to get this far. The preamp part is hard to understand for me, but it seems that the tone controls will turn out to be all active.
I also played a little with the values of R16 and R17, 100k made the input clipping worse, 22k better than the stock 33k and finally 10k gave me the most headroom. I still can get it to clip when I hit the strings real hard, but now that's unlikely in a normal play situation.
So, here's the final schematic.
Are there any obvious design flaws?
Hi Maddus,
Good on you for having a crack at this 8)
but sorry but I doubt it's correct.
You will have to go over this quite a few times to get it right.
My advice is use paper and pencil as you may well end up having to draw it out many times.
Use Nodes to label each wire, then count how many components connect to each node or wire.
I started out doing this well over 30 years ago, no internet just some books and a few schematics. I used to end up with a bin full of scrap paper before I got it right.
Also check pinouts on the active components,wrong pinout on transistors is a common mistake.
Oh ed1; if you can up load some pics of both sides of the PCB we maybe able to help you verify the drawing.
Phil.
Hi Phil,
here are the photos that helped me. The photo from the solder side is already mirrored to allign with the top side.
I also traced it node by node, so I can't imagine I got something wrong.
I think everything is recognizable on the pictures, the transistors with the flat side facing the output transistors are BC212, the ones facing to the rectifier BC182. Under the dual gang pot sits a 4k7 Ohm resistor (R34).
Here is what I get with my pencil-and-paper style
Starmaker 8015 sch.gif
Anyway, so do I have to make the 100k resistor at the positive input of the first opamp against 12V smaller? Doesn't it have to be half the value of the 12V midpoint resistors? So in my case 5k Ohms?
Where did that example come from? Why did they say the biasing resistors should equal R1?
The ratios are about RF and Rin and set the gain of the non-inverting op amp. Your circuit and the example circuits have different gain so the "=1/2 R1" does not apply.
The 100K resistor along with the resistor from the input jack sets the input impedance.
If anything I might make it larger like 220K or 330K. It does not change the gain.
Gain of the first stage is set by the feedback resistor divided by the resistor from
the - input to common.
In this case common is the +12V. The gain of the first stage is 1+47K/2.2K = 22.4
You could change the 100K to 1M but you would want a FET op-amp like TL071 for the first stage
Ok so my options to avoid the (asymmetric) clipping in the input stage are:
1. lower the gain, e.g. with 5.6k in place of the 2.2k resistor
2. lower the series 820R resistor further to get over 24V for the IC supply (now 22V) so the 24V zener diode also finally gets involved.
3. Make the bias resistor pair uneven in value, so that I get a bit a higher than 12V, so that when it clips, it clips to the positive rail instead to cutoff?
If I understood the NE5534 datasheet correctly, the maximum output p-p voltage I can achieve is around 5V lower than the supply. And 0.7V input with a gain of 22.4 is already giving 15.4V just by calculation.
Have you verified that the input stage is the first thing that clips before the amp gets to full power? Seems odd that they would have such little headroom at that stage.
At full power (just before clipping) into a load, what is the peak to peak voltage coming out of the first IC ?
Hi g1,
I scoped the first 5 graphs with a signal generator on the normal input. The blue wave is from in front of the 68k input resistor, like directly at the hot tip of the plug (the voltage at pin 3 is then about half of that value, e.g 700mV in the first image). The yellow is at pin 6 (output) of the first opamp. In the bottom bar you see the Vpp values, at 16V it is beginning to flatten on the negative side.
The riff.jpg is the "smells like teen spirit" riff played with a Les Paul on the bridge Burstbucker 2. There you can see, that I get easily peak voltages that would cause clipping.
To get asymmetric clipping like that makes me wonder if the input to the OP-Amp is biased at 1/2 voltage.
Can you measure the voltage across pin 4 to 3 on IC1, also across pin 4 to 7 and pin 3-7, all with no guitar plugged in.
If it is not set at 1/2 you might be able to tweak the value of R16 and R17 until you get exactly 1/2v at the input at pin 3 on IC1.
You might even have to change the bias slightly off to make IC1 clip evenly.
This is just my thinking and i could be wrong, I think the bias on the input to the OP-Amp determines how the output hits the power rails and flattens out.
If you have a spare 20k pot (or bigger) then connect it to 0v-22v and lift the leg on R20 that goes to the +11v and connect it to the viper on the pot, then you can adjust the bias voltage you have created to see what happens to the clipping on the input stage. (If you can understand my crazy way of explaining it)
Rail to rail clipping is used to produce distortion in some amps just like people use diodes or leds to clip the waveform to produce distortion, it might have been intentional on the amp just to make the sound you don't like but the designer wanted.
EDIT: I did a quick trace of the preamp to see what I got, I used the values already quoted and also the references.(I haven't double checked it or anything)
It looks like a basic baxandall tone stack for Bass & Treble and then the Mid is just like the active Mid in a Lab Series L4 Bass Amp.
Cheers
Mick
I think if you tweak it to get symmetrical you will still overdrive the stage anyway. And it won't sound any better if it's clipping evenly on both sides rather than just one. It's not really meant for that kind of guitar signal.
Some tubes amp do that as well, the gain stage before the volume control clips unless you turn down the guitar volume. Just that with tubes no one really minds as much because the clipping is softer.
You can tweak the gain of the stage to deal with it, but you might just find the next stage or somewhere down the line clips the same way.
That's why I suggested getting full unclipped output and seeing what kind of input signal level that requires.
Thanks for the suggestions, guys.
I don't think that clipping was considered by the designer, because it sounds like someone is lifting the needle off a vinyl record too roughly - like "pbbfffzzttt" ;D .
Last night I simulated the first stage in LtSpice, and the clipping behavior was exactly the same to my measurements.
I played with the values and tried to stick to the scheme in Figure 2 of this:
https://www.analog.com/en/resources/analog-dialogue/articles/avoiding-op-amp-instability-problems.html
But when I use 100k/100k bias resistors in reality, then my 12V ref descends to about 9V. So I got back to 10k/10k, that gave me 11.8V. I also lowered the 820R in the supply to 680R, so I have now real 24V instead of 22V. The gain was also lowered to 19; the screenshot shows the final values I soldered in.
On the o-scope it looks very good now, should be enough headroom. Tomorrow I will test it with guitar and speaker.
I guess what this modding is making so difficult is that the amp was designed for the CA741's and the change to NE5534 brought higher bias and supply currents.
Like someone already wrote, perhaps I should have chosen the TL071.
Tested it, and I still got occasional pops on powerchords >:( .
Now I'm fed up with varying resistor values, I just built a diode limiter and put it at the input before the coupling cap. Now it is rock solid, finally I can windmill my Les Paul and get no unpleasant cracks or pops! I used a 22k pot to make clippig asymmetric on demand, but the effect is more subtle.
Main thing is, that this is now a very good portable pedalboard amp. With that para mids you can dial in any flavour of distortion with just one dirt box in front.