Is this a filter at amplifer output and can I remove it? - diyAudio
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Old 24th July 2014, 01:22 AM   #1
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Default Is this a filter at amplifer output and can I remove it?

I've had this Speakercraft amp for a while and just had the lid off to fix a dry joint. Whilst poking around, I was reminded of some components that don't look in good shape.

There are two stereo sets of binding post on the rear panel. Where the binding posts pass through the chassis, there are two small pcbs with what looks like a resistor and capacitor in series with each other - these are connected across each binding post in parallel.

The resistors look decayed and the cabling near by has melted a little. So something has been hot. The amplifier works fine, but I hope for some advise on whether these components should be replaced or can they be safety removed. I read that output filters can degrade the sound and in some circumstances can be removed.

So is it advisable that these can be removed if they are indeed filters??
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Old 24th July 2014, 02:32 AM   #2
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To help you with specific answers, others need specific details like the model number so the schematic can be searched to find out what it is your pic is showing. Otherwise, posting the relevant part of the schematic or a link to it is good way to get help. Unfortunately, late model schematics are seldom available free on the 'net.

As a guess from the size of the resistor and capacitor, it seems the output filter you refer to is commonly called the Zobel network - usually made up of a 0.1uF film capacitor and around 10 ohm resistor in series from the output (+) to (-) and is a "one size fits all" device. It's not good or normal to fit this remotely from the power amplifier PCB itself but anything is possible from some manufacturers and Speakercraft product is certainly unusual compared to older and mainstream class AB amplifiers, as I understand this amplifier likely is.

If you would confirm the values or markings of those parts, you should have your answer but also a warning of dire consequences for the amplifier's stability if they have failed or you remove them. Expect at least a 100V rated polyester cap and 1W film resistor there, which appears to be the case.

It isn't an option to remove this filter since doing so could lead to the amplifier self destructing. Class AB amplifiers usually do show instability and fairly quickly without a means of damping internal oscillation and interaction with the load, like a Zobel network. If there is damage, replace the parts with like values and ratings but don't omit them.
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Old 24th July 2014, 02:47 AM   #3
kevinkr is offline kevinkr  United States
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I'll second all the above comments. Omit the zobel at your peril..
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Old 24th July 2014, 04:03 AM   #4
Art M is offline Art M  United States
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Some of the reasons for Zoble network component stress are amplifier oscillation
(instability), input signal frequency too high.

Do not run Amp without a functioning Zoble network in good condition.
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Old 24th July 2014, 10:38 AM   #5
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Do not remove safety features on your amp

Damage in the parts of zobel means only a few of things

One, that you make excessive use of high in your set up ( treble is always at quiet big level ) meaning that either your speakers or your ears are not happy with a reasonable amount of high

Two, in the installation there is a flaw that decrease the bandwidth of the signal> forcing you to add more high > forcing the zobel

Three, speaker cables are too long , two wrong ,and/or load is too capacitive and zobel struggles to suppress the products of this installation

Replace the worn parts but most of all find out what is wrong

Kind regards
Sakis
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Old 28th July 2014, 10:08 PM   #6
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Thank you all for your replies.

Ian you were right on many counts...

I've hunted for a schematic on the web and turned up nothing. I tried calling Speakercraft technical department; they were unable to provide a schematic or offer advice on component values required.

I had another chance to look inside it and have a few more details.

The amp is a Speakercraft - BB2125.

At the output speaker terminals, the capacitors are marked: IF 0.047k PMD 160V

The resistors are so badly damaged that I am unable to read them though can see a red ring and a gold ring on the outer edges of one resistor.

I tired measuring the resistors (in circuit) and read the following:

Speaker A Left: 20M Ohms
Speaker A Right: 15 Ohms
Speaker B Left: 26 Ohms
Speaker B Right: 2.4M Ohms

Quote:
Originally Posted by east electronics View Post
Do not remove safety features on your amp

Replace the worn parts but most of all find out what is wrong
OK, I would like to keep the safety feature and repair if the parts if possible. Though as above I am unsure o the value of resistors (should they all read 15Ohms which is close to the value predicted by Ian)?

As for what is wrong, the amp came to me in this condition but seems to play fine. There are only a few mV of DC on the speaker terminals. But on the main amplifier board there is another pair of filters (or similar components) that are also showing similar signs of overheating. -I've attached a picture in the pdf.

This other filter is located on what looks like the main DC rail power supplies near where they connect to the main amplifier PCB.

Surely too much coincidence that identical looking components else where on the board look smoked too? Would this indicate a power supply issue?
Attached Images
File Type: jpg Overview.jpg (322.2 KB, 85 views)
File Type: jpg Out put filter.jpg (241.3 KB, 78 views)
Attached Files
File Type: pdf SpeakerCraft BB2125 - resistors.pdf (380.6 KB, 6 views)
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Old 28th July 2014, 10:32 PM   #7
sreten is offline sreten  United Kingdom
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Hi,

It would be very odd for the Zobel to be affected by A/B
speaker switching and no way are your readings across
the resistors for A and B correct without a rather large
reassessment of what they are intended to do.

e.g. 20M across an output resistor = open fried,
15R across the same resistor switched = that
another parallel component must now be involved.

A/B switching is very unusual, usually A or A+B
(series or parallel), or A / B / A+B (S or P).

rgds, sreten.
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Last edited by sreten; 28th July 2014 at 10:37 PM.
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Old 28th July 2014, 10:39 PM   #8
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The zobel network is there to provide a flatter response for the amplifier at higher frequencies. The speaker impedance goes up at higher frequencies and the zobel compensates for this giving a more constant load across the audio band.

I have never fitted one to stop oscillation in my designs.
If there is oscillation it can usually be stopped at the VAS stage with a VAS transistor B-C capacitor of around 100pf.

I recently overhauled a Maplin 225WRMS amplifier.
I fit brand new transistors on the outputs and the amp oscillated badly.
I checked the new transistors on a transistor tester and the gain was huge compared to the spec of the original transistors.
I fixed the problem by slightly increasing the capacitor in the VAS stage.

Last edited by nigelwright7557; 28th July 2014 at 10:41 PM.
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Old 28th July 2014, 10:50 PM   #9
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Thanks Sreten.
Quote:
Originally Posted by sreten View Post
Hi,

It would be very odd for the Zobel to be affected by A/B
speaker switching and no way are your readings across
the resistors for A and B correct without a rather large
reassessment of what they are intended to do.

A/B switching is very unusual, usually A or A+B
(series or parallel), or A / B / A+B (S or P).

rgds, sreten.
There is no A/B speaker switch, both speaker outputs are wired in parallel from the same point on the main board. There is a 4 or 8 ohm speaker selector though.

Quote:
Originally Posted by sreten View Post
20M across an output resistor = open fried,
15R across the same resistor switched = that
another parallel component must now be involved.
So the high reading resistors are likely to be caput?

All the resistors look damaged and some caps are showing signs of heat from the resistors.
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Old 28th July 2014, 11:02 PM   #10
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Quote:
Originally Posted by nigelwright7557 View Post
I have never fitted one to stop oscillation in my designs.
If there is oscillation it can usually be stopped at the VAS stage with a VAS transistor B-C capacitor of around 100pf.
Presumably the zobel is not now functioning as it should even though sounds are still coming out. Not sure about oscillation, if I can find out how to check for it I could dig out the scope to see.
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