Hi,
I am fixing this an old amp for a friend
https://drtube.com/schematics/marshall/vs65-61-02.gif
The speaker was busted due a cold soldering
on the +/- leaving the final with one power, and somehow
the speaker took about 35V+ and got busted.
No other components were damaged, a full re-flow with the pipe torch fixed the problem. I am thinking to protect the speaker with a fast fuse 3.5A > 100w or a non polarized capacitor 1000uf/22v.
Which way to go.
- would the cap affect/dammag the idle if no DC flows to speaker?
-would the fuse if blows would damage the map without the 8ohm load
Thank you.
I am fixing this an old amp for a friend
https://drtube.com/schematics/marshall/vs65-61-02.gif
The speaker was busted due a cold soldering
on the +/- leaving the final with one power, and somehow
the speaker took about 35V+ and got busted.
No other components were damaged, a full re-flow with the pipe torch fixed the problem. I am thinking to protect the speaker with a fast fuse 3.5A > 100w or a non polarized capacitor 1000uf/22v.
Which way to go.
- would the cap affect/dammag the idle if no DC flows to speaker?
-would the fuse if blows would damage the map without the 8ohm load
Thank you.
It would be hard to get the fuse right for a speaker.
Too high and it never blows and damages the speaker.
Too low and you get nuisance blowing.
The 1980's Maplin 225WRMS amp had a speaker fuse and it saved the amp once when I tripped over the speaker and it pulled out and shorted. A quick fuse change and off I went again.
Too high and it never blows and damages the speaker.
Too low and you get nuisance blowing.
The 1980's Maplin 225WRMS amp had a speaker fuse and it saved the amp once when I tripped over the speaker and it pulled out and shorted. A quick fuse change and off I went again.
A 2A fast blow fuse would be very safe. If you use a series capacitor, it should be
more like 10,000uF at 50V non-polar. Even if you end up using a smaller capacitor,
don't use less than 35VDC parts.
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Fuses aren't going to protect anything. A capacitor protects from DC, but it must be large. I believe 10.000 uF is too much. Try 50 uF upwards and listen if You have enough Bass.
For tweeters the are light bulbs and poly switches, or even the Chinese Mosfet Shunt circuits.
Better is to install a speaker protection circuit after the Amp.
For tweeters the are light bulbs and poly switches, or even the Chinese Mosfet Shunt circuits.
Better is to install a speaker protection circuit after the Amp.
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Into 8 ohms, with a 2Hz corner frequency (needed to minimize distortion
from the electrolytic capacitor), the value of C = 1 / (2Pi x 8R x 2Hz) = 9952uF
A 50uF capacitor would result in a corner frequency of 400Hz, which is clearly inadequate,
and also very high in distortion due to signal voltage appearing across the capacitor.
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It is a guitar amp. 80Hz may be enough. However g-speakers typically resonate near here, with impedance to 50 Ohms. 47uFd or 100uFd may be "OK". It will skew the tone around the lowest open-string note, but not necessarily "wrong".
A cap that size has to be electrolytic. In this circuit, it has to be a BiPolar, as for loudspeaker crossover.
But what are you doing? The real problem was a "cold solder". I am sure you have inspected all the joints, and cleaned and re-soldered any doubtful blobs. So it "should never go bad again". I had a Bogen with a factory NO-solder, hanging by the crimp. It caught fire. Repaired the screw-up and beat it for years with no problem.
A cap that size has to be electrolytic. In this circuit, it has to be a BiPolar, as for loudspeaker crossover.
But what are you doing? The real problem was a "cold solder". I am sure you have inspected all the joints, and cleaned and re-soldered any doubtful blobs. So it "should never go bad again". I had a Bogen with a factory NO-solder, hanging by the crimp. It caught fire. Repaired the screw-up and beat it for years with no problem.
I would look at the bigger picture. You are probably replacing the Darlington power transistors, which probably blew when a speaker cord shorted. You can add a simple current limit that may prevent this kind of failure by adding a couple 1n4148 diodes (reverse biased) from the Darlington bases to "SPKR".
Then, you could add a speaker relay board like many here in DIYA posts, ie in the amp.
Any capacitor is going to seriously affect the sound and "bipolar" caps anywhere big enough are going to be a pair of polar caps, maybe plus diodes. A serious sound quality issue and expense.
The best capacitor solution is the "QSC" circuit, ie add two (polarized) caps in series from LT- to LT+ and connect the speaker return to the mid-point connection between caps but here that is complicated by the headphone jack and the current feedback in the speaker return path.
Likewise a fuse is either not going to protect fast enough and/or fail all the time.
I really think adding a speaker relay board is the way to go.
Then, you could add a speaker relay board like many here in DIYA posts, ie in the amp.
Any capacitor is going to seriously affect the sound and "bipolar" caps anywhere big enough are going to be a pair of polar caps, maybe plus diodes. A serious sound quality issue and expense.
The best capacitor solution is the "QSC" circuit, ie add two (polarized) caps in series from LT- to LT+ and connect the speaker return to the mid-point connection between caps but here that is complicated by the headphone jack and the current feedback in the speaker return path.
Likewise a fuse is either not going to protect fast enough and/or fail all the time.
I really think adding a speaker relay board is the way to go.
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