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Old 1st June 2002, 05:40 AM   #11
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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.
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Old 1st June 2002, 06:27 AM   #12
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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 ?
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Old 1st June 2002, 08:17 PM   #13
Geoff is offline Geoff  United Kingdom
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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.

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Old 2nd June 2002, 02:28 PM   #14
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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?.
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Old 2nd June 2002, 02:51 PM   #15
Geoff is offline Geoff  United Kingdom
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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.

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Old 6th June 2002, 02:48 PM   #16
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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?
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Old 6th June 2002, 04:53 PM   #17
Geoff is offline Geoff  United Kingdom
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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.

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