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Old 30th May 2002, 07:34 PM   #1
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i built 2 JLH amps. Great amp. However there is a hum problem. Sometimes the amp will just out of the blue emit a hum signature of bzzz coming from the tweeter. THis misfortune comes and goes randomly. Normally the amp will be quiet when powerup. I initally taught that my supply line is dirty so i install a
LC audio noise filter. Still the problem is there. Can anybody help on this matter. I have install 22nf across by diodes but i didn't place any film cap bypass across the powercaps. could this be a remedy?
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Old 30th May 2002, 08:25 PM   #2
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What did you use as output transistors? 2n3055 or something else.
I ask this because I had a similar problem with a DOZ when I used differerent output devices.

Regards
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Old 30th May 2002, 08:29 PM   #3
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What kind of power supply are you using? I built a cap-x psu as suggested by Mr Elliot and i'm having no problems with hum or noise.
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Old 30th May 2002, 08:47 PM   #4
rljones is offline rljones  United States
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It may be garbage coming through the ground connection, such as someone running a fluorescent light that is on a nearby circuit.

One easy method is to try a 'cheater' plug (converts 3-pronged to 2-pronged) on the preamp to try and break up a ground loop. Another method to be carried out inside your amps is to not connect the 3rd prong (the ground) to the power supply. Do, however, connect it to the chassis at some point for safety. This allows the power supply to float and will break up a ground based hum. Another technique is to take the floating power supply ground and connect it to the separate chassis ground just described using a 10 ohm/5W resistor. This probably provides a greater safety margin that simply floating the amp's power supply.

Also, you might, if you haven't already, install a 0.47 microfarad/400V cap across the AC inputs on the primary side of the transformer in your amps. This can reduce some noises.
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Old 30th May 2002, 09:18 PM   #5
Geoff is offline Geoff  United Kingdom
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Nick

You describe this problem as an intermittent bzzz coming from the tweeter so I think it is unlikely to be caused by power supply noise or an earthing problem (unless you have some severe interference on the mains with which your filter is unable to cope).

There are various possible causes for an intermittent fault, including a dry joint that breaks as the amp warms up or an oscillation under particular load conditions. Before I can be more specific, I will need some additional information about your amps.

Does the problem occur in both channels or only one?

Which version of the JLH have you built?

What type of transistors have you used, particularly Tr1, Tr2, Tr3 and Tr5 (if 1996 version)?

How long are the leads to Tr1 and Tr2?

If you have built the 1996 version, what device did you use for the 15V regulator in the Tr4 emitter circuit?

How did you construct the amps, pcb or point-to point wiring?

Do you have on-board supply rail decoupling?

What supply rail voltage and quiescent current are you using?

Have you earthed the heatsinks?

What load impedance do your speakers present? Are they electrostatics?

I will probably think of some more questions, but those will do for now.

Geoff
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Old 31st May 2002, 03:05 AM   #6
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i built the 69 version with dual supply
2sa872,bd139,mj15003
tr1 leads are 20-30cm,
tr2 cm are <10 cm
point to point
install only 100uf no 100nf decoupling caps
+- 22
heatsink are at 130 Farenheit in celcuis i not to sure
2.1amps Iq
heatsinks are earthed
load impedance at nominal are 8 with 90db(conventional speakers)
Power supply are 20000uf-0.5ohm-44000uf forming a CRC
No decoupling caps were used
method of connecting ground was
power supply was stargrounded
amp circuit is stargrounded
ps star to amp star then connected to input ground
which was connected to chasis ground
input ground and chasis ground are the same potential as
I didn't insulate the phono so it touches the metal of the chasis.
Power supply is floating now.
No connection to mains earth
At the IEC plug installed several 100nf and varistors from live and neutral. live and ground and neutral and ground
I read somewhere in tnt audio he install a 9.1ohm with a 220nf rc network before the transformer does this work.
also he added that the ps should included 100 uf and 100nf for better filter for rf collected along the way
another rc network is used that is 1 ohm and 680nf for getting rid of any residual capacitor inductance
can any of this work for my benefit?
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Old 31st May 2002, 12:24 PM   #7
Geoff is offline Geoff  United Kingdom
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Nick

I'm somewhat clearer now as to what you have built. From feedback I have received, the 1996 version is more prone to oscillation problems than the 1969 so this is one thing in your favour.

The transistors you have used shouldn't be causing a problem, assuming the MJ15003s are genuine and not some of the counterfeit ones that appear to be rife. Equally, I cannot see any obvious errors in the rest of your construction.

The connections to Tr1 are a little longer than I would ideally like, but should be OK provided the three wires are run close together (either twisted or tied every few cms). Your earthing arrangement seems alright and anyway a problem here would be continuous, rather than intermittent, unless a dry joint was involved.

You didn't say whether the problem was in both channels rather than just one. Also, are you sure that it is the amp that is at fault rather than a fault in the tweeter itself? If you are certain it is the amp then I suggest the following:

Check the authenticity of the MJ15003s. There is a thread on this forum that gives details of genuine and counterfeit devices.

Check that the input stage is well separated from the output stage and associated wiring (at least 50mm).

Ensure that the input and ouput cabling is well segregated.

Check that the electrolytics have been connected with the correct polarity.

Check for dry joints.

If you don't find a fault and the problem persists then I would try the following (one step at a time, monitoring the result at each stage):

Fit a Zobel network (100ohm in series with 100nF) across the amplifier output terminals.

Fit a base-stopper resistor (about 100ohm) directly to the base of each output transistor.

Fit 100nF supply rail by-pass capacitors close to the output stage.

Fit the 4k7/330pF input filter from the 1996 version in case the problem is being caused by rf pick-up in the input cabling.

As a last resort, connect a 100pf capacitor between the collector and base of Tr3. If this solves the problem you definitely have an intermittent oscillator rather than an amplifier.

Most of the foregoing assume an intermittent oscillation though, as others have said, the problem could be due to interference or a noisy ground.

One final question, what circuit arrangement did you use for controlling the dc offset?

Geoff
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Old 31st May 2002, 02:24 PM   #8
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You used MJ15003 as output devices in your JLH 69. I used 2sc2922 in a DOZ and had similar problems. That's why I asked in my previous post.

A zobel (100 ohm resistor and 0,1uF capacitor) mounted on the output terminals of the amplfier solved my problem.
This is easy to start with and test. If this does not solve the problem, try the other excellent suggestions that Geoff made in his post and let us know.

Regards
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Old 31st May 2002, 02:40 PM   #9
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hello

rtirion:

i build Doz too and sounds good but i would like to upgrade it.

are 2sc2922 sounds better like originals or mj 15003?
what is your power supply voltage?
how much bias current do you use?
do you make any other modifications?

are you happy with sound of your DOZ.

janey
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Old 1st June 2002, 12:01 AM   #10
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Janey,

- are 2sc2922 sounds better like originals or mj 15003?
Yes, the DOZ sounds better to me with 2sc2922 then with 2n3055. I can't comment on mj15003 because I did not use them.
I also tried 2sc3281. Sounds is on pair with 2sc2922. But I did have thermal problems. No oscillations

-what is your power supply voltage?
42 V

-how much bias current do you use?
2.2 amps. Needs very large heathsinks!! Each device dissipates around 40 watts. Mount the 2sc2922 very carefull.

-do you make any other modifications?
I have used a power supply with CLC configuration. 37mF-2.2mH-37mF. The coil is a really large one, 2.5mm wire, air coil. Used a 35A metal can bridge on a sink, 225va transformer. Each channel has it's own supply.

I have used OSCON electrolytic caps in the amp.
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