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[NickC]

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?

 

[rtirion]

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

 

[Lisandro_P]

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.

 

[rljones]

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.

 

[Geoff]

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

 

[NickC]

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?

 

[Geoff]

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

 

[rtirion]

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

 

[janey]

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

 

[rtirion]

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.

 

[Lisandro_P]

Speaking of oscilations... the 7815 that's used as a current source for the dc offset control has been known to oscilate and make hum appear at the output. Try a 4,7k or so resistor from the output of the regulator to ground and increasing the input and output capacitance. OR, even better; replace it by the discrete ccs proposed by Geoff. I never had problems with the 7815 myself, but i'm using the discrete version and works great.

 

[NickC]

geoff the hum is in 2 channels. THe channels are built in mono. The thing that puzzles me that hum just comes and goes.

The long wires were not tired together or braided is this a main problem

The transistor are high chance of being orginal. Bought from former motorola distributer.

The input and output cables are separated by >5 cm

The feedback capacitor c5 220uf as the positive side to ground.
The zobel network is similar with project 3a of esp rite?
plus what's the purpose of the zobel network?

for the base stopper resistor of 100 ohm. The connection of the resistor is from base connection in the circuit to the transistors base. Am i correct? I presume this resistor also has a rating of above >5 watt. What the use of the base stopper resistor?

100nf supply decoupling caps is to prevent the amp from oscillating. what material is best here? ceramic/polyester/ polyprop?

4k7/33pf forms a low pass filter

the 100pf works like a miller domination cap

the dc offset control is the same in the 2 supply version of the 1969. a pot in series with 68k.

Geoff I posted several statements above from my point of understanding. Please feel free to correct it. THe whole purpose of posting it.
Can i also increase the Iq to 3-4amps without changing the circuit ?

 

[Geoff]

Nick

In your original post you referred to a Bzzz from the tweeter, rather than a hum which would be much lower in frequency, which is why I am persuing the possibility of an intermittent high frequency oscillation. Due to intermodulation effects, a high frequency oscillation can cause noise within the audible frequency range.

If however you are experiencing a definite hum (50 or 100Hz UK, 60 or 120Hz US etc) then you need to look elsewhere. Being intermittent, I would suggest a dry joint or faulty resistor/capacitor. In the absence of further information on this, I will proceed with the assumption of a hf oscillation.

Answering/confirming your queries/comments:

Keeping the output transistor leads close together prevents the formation of a loop which could cause oscillation.

The Zobel network provides an increasing impedance to the output stage as the frequency rises which improves amplifier stability particularly into inductive loads. It is indeed as shown in Project 3A.

You are correct about the base resistor. It is connected in series with the lead to the base, at the transistor end of the lead. Its purpose is the same as the gate resistor in MOSFET circuits, to reduce the possibility of oscillation in the output devices (as opposed to Nyquist oscillation of the whole circuit). MOSFETs need this resistor as they have a very high ft. Some BJTs require it for the same reason or to overcome defective layout or wiring. The MJ15003 should not need a base resistor since it doesn't have a high ft. However, if the base resistor cures the problem, it indicates that either the transistors are out of spec or the laout/wiring needs some alteration.

100nF decoupling caps. I use polypropylene, ceramic are best from the point of view of impedance. Use whatever you have available (anything between 50nF and 500nF) just to provide a lower impedance path to earth at higher frequencies.

Yes, the 4k7/330pF form a low pass filter at the input (-3dB at about 100kHz) which reduces the amount of rf entering the amp.

Again yes, the 100pf cap creates a dominant pole which causes the high frequency gain of the amp to roll-off earlier, increasing the Nyquist stability. If this cap cures the problem, it again indicates that you have a component or layout fault.

The quiescent current can be increased to 3-4A but you will need additional output transistors, some 0R1 emitter resistors and higher wattage resistors in the bootstrap circuit. See the 'JHL for ESL57' article on my website for details. Note, if you decide to do this, the optimum supply rail voltage and quiescent current will depend on the impedance of your speakers.

Geoff

 

[NickC]

i do have a 50- 100 hz hum i think but not that loud. I think this comes from the unregulated power supply. I order some chokes for 5A for the PS. I used the zobel network, but the sound is still there but slighty softer now.

I will get some ceramic caps for decoupling. I will used to decoupled the amp stage. Can i used ceramic caps for decoupling of power supply capacitors. I heard it is best to used film type.

i plan to upgrade this amp power to 25 watts. with 22 volts how much of Iq should i increase it to?.

 

[Geoff]

A quiescent current of 2A will give you 25Wrms into 8ohm. You will only need to increase Iq if you want this power into a lower impedance load.

The peak output current of the JLH is about 1.3 x Iq so for other load impedances, calculate the peak output current from:

Ipk = sqrt (2 * power / impedance)

then divide by 1.3 to get Iq.

Geoff

 

[NickC]

Thanks for the contributing to my post, especially geoff
great help
geoff for power output of the amp you mention in the last post how to calculate the power of the amp
but this way calculated with a known Vcc
let say i increase the voltage above 20volts to 25 volts how do i calculate the power then?
is there a generic formula for calculating power of the amp i can used?

 

[Geoff]

The peak output voltage (Vp) required for particular rms power output (Prms) and load resistance (RL) can be calculated from:

Vp = sqrt(2) * sqrt(Prms * RL) = sqrt (2 * Prms * RL)

For the 1969 version of the JLH, the minimum supply rail voltage (Vs) must be:

Vs = 2 * (Vp + Vcesat)

where Vcesat is the collector-emmitter saturation voltage of the output transistors at the peak output current.

For a dual-supply rail version of the 1969 circuit, each rail needs to be (Vp + Vcesat). Vcesat can be assumed to be 1V in the absence of datasheet infomation.

The quiescent current control circuit of the 1996 version is less efficient than the bootstrap arrangement in the 1969 design and requires a supply rail approximately 4V above Vp, ie:

Vs = Vp + 4

The current calculations were given in my last post and are common to both versions.

I will leave it to you to rearrange the equations to determine the output power available from a given supply rail voltage or quiescent current.

Geoff