| bonsai171 |
Hello, i recently built a small preamp kit, and after bulding it, and firing it up, it seemed to have a tremendous amount of distortion. The distortion is plainly audible, and after checking the connections, i noticed that the ground isn't wired correctly.. Could this be the source of the problem?? Or is there something more? What would be the best way to track down this problem?? Tia
Dave |
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| HDTVman |
By all means fix the grounds. That definitely can be causing you trouble.
later BZ |
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| bonsai171 |
well today i sat down and set up the grounds properly (i think??) But unfortunately there is still massive distortion:( What's the next step in trying to fix this problem?? tia
Dave |
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| HDTVman |
Do you have any test equipment? A signal generator, volt meter and oscilloscope are very handy to have at this point.
Later BZ |
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| bonsai171 |
I don't think i have any of that stuff, but there's a possibility of a voltmeter.. What are the options at this point?? tia
Dave |
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| HDTVman |
Well Dave, I or anyone else for that matter need to know more about the circuit. Just what preamp is this, a circuit diagram would help a lot. A disciption of what the distortion sounds like would help too.
You need some way to make a few measurements to find out what is going on with the circuit. With out that anything I or anyone else says would just be a wild guess.
Later BZ |
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| bonsai171 |
| Hello, here is the schematic for the preamp. Also, I just found the voltmeter, so hopefully this will be helpful :) Thanks |
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| bonsai171 |
Here is the what the distortion sounds like- when you first power up the preamp, everything is quiet, and then if you start the music, it plays fairly cleanly for a few seconds. After that, the sound becomes very garbled and disgusting sounding. Hope this helps! Tia
Dave |
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| Magura |
I once had a similar problem. It was caused by a dead capacitor.
Magura:) |
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| bonsai171 |
Hi, if there is a dead capacitor, how would it be tracked? There aren't too many of these, so even if it required some kind of testing for each of them (there are 12) it wouldn't be bad :) Thanks!
Dave |
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| AKSA |
Check collector voltages wrt earth.
C of Q2 should be around 8V; C of Q1 should be around 4V.
Then measure the voltages across the emitter resistors and calculate the current flowing by Ohms Law.
Do this at no signal immediately on switch on, and follow the voltage carefully for the first ten minutes. If it changes after a short time, it's likely to be a cap. First one to look at would be C1, then C3, then C2. Often it's easier to just swap them out and see if it makes a difference. Remember, you are finding a fault quickly, not agonizing over methodology.
Cheers,
Hugh |
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| Magura |
Those are all relatively small caps=cheap caps. Id simply swap them as AKSA wrote. Its the fastest way to eliminate the dead cap option.
Magura:) |
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| bonsai171 |
Which capacitors should be replaced, the electrolytic ones, or the ceramic disc ones? A friend of mine also mentioned to me that these are very heat sensitive, and since i have a high wattage soldering iron they could have just been melted:( What is a good wattage iron to work with for soldering these? Also, if this preamp does turn out nice, which capacitors could be upgraded to nicer ones later on?? tia
Dave |
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| HDTVman |
How high wattage is your iron? 15 to 30 watts should be plenty for what your doing. Transistors can easly be damaged by too much heat as well as film caps.
Have you taken those voltage readings yet?
Later BZ |
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| john curl |
| Caps in backward? |
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| bonsai171 |
Hdtvman,
Are you testing the actual voltage at the transistors themselves, or the capacitors after them? If so, which capacitors do you mean? How exactly would you do this- put the positive lead and negative lead from the multimeter on each end of the capacitor? Also, there are different voltage settings on the multimeter. Since the power supply for this project is 12v dc, would i use one of the dc settings? There are a few choices: 60v, 12v, 3v, 0.6v.. thanks! |
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| bonsai171 |
The wattage of the soldering iron is 25W
Dave |
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| DRC |
but how are Q1/Q3 biased ????
EDIT
I think I see now :clown:
Are you feeding it with too big a signal ?
Dave |
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| bonsai171 |
The input of the preamp is being fed by a Rotel Rcd-02 cd player, which is a 2 volt signal. The funny thing about the schematic for this is that there is no volume control in this, so i'm beginning to wonder if i'm using this circuit for the wrong purpose. The manual itself calls it a "stereo preamplifier" , and here is what it says for uses: "This pre-amp can be used to boost speaker sound on conventional cassette or record players where unamplified speakers are too weak to deliver audible sound. Some of its features include Stereo and Auto Gain Control. Output can be directly connected to a set of earphones." Any thoughts??
Thanks!
Dave
In other words, is this meant for driving a power amplifier?? |
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| bonsai171 |
John Curl,
All of the capacitors are facing the right way- the negatives are hooked up to the ground :smash: thanks
Dave |
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| HDTVman |
| quote: | Originally posted by bonsai171
Hdtvman,
Are you testing the actual voltage at the transistors themselves, or the capacitors after them? |
The transistors. We need to know what the voltage measures with the negitive meter lead to ground. The positive lead should then be connected to the emitter of each transistor, then the base of each and the collector of each. With these voltage measurments it can be determined if the circuit is working correctly.
| quote: | | Also, there are different voltage settings on the multimeter. Since the power supply for this project is 12v dc, would i use one of the dc settings? There are a few choices: 60v, 12v, 3v, 0.6v.. thanks! [/B] |
The 12 volt DC range for starters then lower ranges if the reading is very near the bottom of the meter scale.
| quote: | [i]Originally posted by bonsai171
The input of the preamp is being fed by a Rotel Rcd-02 cd player, which is a 2 volt signal. The funny thing about the schematic for this is that there is no volume control in this, so i'm beginning to wonder if i'm using this circuit for the wrong purpose. The manual itself calls it a "stereo preamplifier" , and here is what it says for uses: "This pre-amp can be used to boost speaker sound on conventional cassette or record players where unamplified speakers are too weak to deliver audible sound. Some of its features include Stereo and Auto Gain Control. Output can be directly connected to a set of earphones." Any thoughts??
Thanks!
Dave
In other words, is this meant for driving a power amplifier?? |
From your discription I could just be a headphone amp. To use it with the CD player you have, it will need a volume control before the preamp.
Later BZ |
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| bonsai171 |
Here are the voltages of the various capacitors:
c1: 1/2 volt
c2: 1/2 volt
c3: 5.5 volts
c4: a voltage drop
c5: 5 volts
c6: 5.5 volts
c7: 1 volt
c8: 5.5 volts
What do you make of the voltage drop at c4? That seems kind of strange. It seems like the voltage of c2 and c4 should be the same (are they both part of the first gain stage??)
Dave |
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| rwbrown1 |
(just a thought)
Is the problem occuring on both channels or just one? You might try disconnecting the power from one channel at a time to see if the problem exists in both channels or just one. That might help isolate the problem.
A scope would be nice but I'm guessing that one is not available. |
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| bonsai171 |
Hdtv man, here are the voltages for each part of the transistors:
Q1:
Emitter: 1/2 volt
Collector: 3.75 volts
Base: 1.5 volts
Q2:
Emitter: 2 volts
Collector: 25 volts
Base: 3.5 volts
Q3:
Emitter: 1/2 volt
Collector: 3.75 volts
Base: 1.5 volts
Q4:
Emitter: 2 volts
Collector: 25 volts
Base: 3.5 volts
Rwbrown:
Both channels are equally distorted.
Hope this helps! Thanks again guys :D
Dave |
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| rwbrown1 |
Strange... Your schematic shows C2 and C4 with different polarities and I believe that C2 shows the correct connection. I really don't know if this is a problem but it does not look correct to me. Most circuits that I have seen have the + side of the electrolytic on the amp side and the - connection going to the input connector. There may be more knowlegable people out there who can comment on this. (I am not an EE or even close)
Could this have caused the strange reading on C4?
(ps. I suggested trying one channel at a time since I have seen cases where the bad side caused both channels to distort. Hooking up one side at a time isolated the bad one.)
Don't give up yet! |
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| bonsai171 |
rwbrown,
yes that does look really strange! If c1 and c2 are the first gain stage, it would make sense that each of the capacitors would have an equal amount of gain. If you look at the very last stage of the preamp, there is also a slight difference in voltage between those capacitors too (c3 and c5), which seems to show that there is a slight loss in dB on the right side. The instructions in the kit show that the c4 cap is supposed to be hooked up this way, but it doesn't seem right at all:whazzat:
Also, i tried hooking up just one channel of the input and the same of the output, first for the left channel, and then for the right channel. In both cases, there is still massive distortion..
Dave
p.s. The idea of switching that cap around is very tempting!! |
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| rwbrown1 |
The plot thickens.... Take a look at c11 - c12 and r4
now take a look at c13 - c14 and r7 ...
You'll notice that capacitors in the same position are not the same value(?!). c11 does not = c13 and likewise c12 does not = c14 ! The values of those puppies are reversed in the diagram between the right and left channels. The question thus becomes -> "Why?"
We need someone much better than I at these sorts of things to say which one is correct. Those channels should be identical(I'm pretty sure of this but don't really know which side is correct.) |
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| slowhands |
| quote: | Originally posted by HDTVman
.... To use it with the CD player you have, it will need a volume control before the preamp.
Later BZ |
Hello, did you read this? You are grossly overloading the preamp by driving it with your CD player. Put in a volume control........ |
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| bonsai171 |
Hello,
Thanks for pointing that out. At the moment there is a volume control on the preamp, but it is after the output circuit. Is it normal to place the volume control directly after the rca inputs?? That seems similar to the way a passive preamp works- by attenuating the input signal, and then sending it to the amp.. In any case i'll try it!
Dave |
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| bonsai171 |
hello,
The volume control before the preamp definitely didn't work. I almost blew my 8" woofers out doing this. Everything is definitely wired up right, but a huge amount of hum and distorted signal came out of the preamp when the amp turned on. I have a loaner tube preamp from a local audio shop at the moment, and it is playing some music for me now. Luckily everything is ok with the speakers and equipment.. i think it's time to throw in the towel on this project.. building components isn't for me. Fortunately the kit was very inexpensive..
Dave |
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| hjelm |
The capacitors C2 C4 must have the positive side pointing at the base of the transistor.
The thing that bothers me is the 25v at the Q2 and Q4 collector. This i think indicates one of two things.
The PSU delivers too much voltage or you have some oscillations in the circuit. |
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