Chapter 3: How to build with examples

[alexcp]

[h=Chapter 3: How To Build With Examples ]%1[/h]

The following comments are hints from people who have built up one of these boards.

[h=Soft Start Circuit ]%2[/h]

Many have suggested that a "soft start" type of circuit will be necessary to avoid a large surge upon power on.

There are a number of threads that relate to soft start circuits.

• [First thread] See circuit in post #7
• [Second thread] A circuit based in Rod Elliot's Project 39, Figure 2 (finally working in the later posts, final version
• [here]) Also See [Elliot Project 39].
• [Third thread] How to wire a soft start with the power supply.
• [Fourth thread] A recent article on inrush limiting.
• [Post#4323] Indicates that Algar Emi has a soft start circuit up for a group buy on this [Fifth thread].

Note: The use of a CL-60 thermisitor (such as Digikey part no. KC006L-ND) in-line with the big transformer primary works fine for inrush limiting. Use one per channel assuming dual mono. This is the same thermisistor used in the Pass amps. The thermister will get hot during operation so leave room for and account for this. Also, because the thermistor will be hot during operation, this technique offers no protection to a on...(long time)...off...(short
time)...on cycle because it needs time to cool in order to get its resistance back up.

If no method is inrush limiting is utilized one will definately blow the bridge rectifier!

[h=Testing The Driver Board ]%2[/h]

You don't need outputs to test the board. The drivers are loaded by their emitter resistors which need to be present...the bias should be set to give about 1.2v across each of the resistors.

The gain of the amp is set by r130 divided by r129, or about 14.6. So you will need a little over 2v to get it to full output. The KSA100 had higher gain in proportion to it's higher output voltage potential.

If your front end is working like mine you will get the following voltages (measured across the identified resistor):

• r103, r107 (rail-0.6v)
• r108, r109 (rail-27v)
• r112, r113, r116, r117 ~2.2v
• r120, r121 ~1.59v
• r122, r123 ~1v

Obviously the junction of the driver emitter resistors should be somewhat close to 0v, but may need some trimming with r105

Resistor tolerances can make all of these vary some, but probably not more than +-25%, or at least if component tolerances are far enough out to make the voltages vary more, consider selecting other components...

[h=Output Transistor Choice ]%2[/h]

in [Post 6725] Stuart Easson writes:

The specific choice of transistors for the output is not without some controversy. Here's my synopsis of the threads 'findings':

mj15003/4: The original amp used a pair each of these, per channel. Very rugged, but sort of slow, technically there are now better transistors. Not available in the plastic packages, so you need to be good at drilling or have pre-drilled heatsinks. With transformers in the 300-400va per channel 2x of each are more or less bulletproof. Bigger transformers would suggest more paralleled transistors, since the rails won't collapse as low impedance loads are driven and the transistors would be at risk...

mj21193/4: The closest 'modern' transistor to the originals, much better gain linearity, more package options, easier to use. In the TO3 form can be substituted pretty much 1-1, in plastic package the ratio is probably better kept at 2-3. Since they are not much faster than the originals instability is not likely any more of a problem.

mj4302/4381: Very much faster, technically a much better transistor, AFAIK not available in TO3, so more are needed for an equally bulletproof output stage, 3 pairs per channel for instance. These transistors are very much faster than the originals and I'd exercise care in the output stage wiring, better to be safe than sorry.

The other transistors you mention (2SA1943 /2SC5200) are more or less unavailable, but there are a lot of fakes on the market, so the consensus seems to be avoid them and use the modern equivalents.

I have tried the mj15003/4 and the 21193/4 and they both sound excellent to me...I think the amp is probably 'better' into low impedance loads with the 21193/4 because of the better gain at high currents loads the drivers less, and it's easy to use more of the plastic outputs.

[h=Transistor DC Measurements ]%2[/h]

Mark measured the DC components of his board in [Post #3067]. They are with reference to ground and keep in mind the rail voltage is +/- 39 volts DC

border="1"
|-
| Q-101 || E .745 || B 142.9 mv || C +36.07
|-
| Q-102 || E .442 || B 143 mv || C - 35.6
|-
| Q-103 || E .617 || B 0 || C +35.9
|-
| Q-104 || E .320 || B 0 || C -35.75
|-
| Q-105 || E +36.5 || B 35.95 || C 0
|-
| Q-106 || E -36.32 || B -35.7 || C 0
|-
| Q-107 || E +37.05 || B +36.5 || C +1.29
|-
| Q-108 || E -36.8 || B -36.33 || C -1.288
|-
| Q-109 || E +.728 || B +1.296 || C +38
|-
| Q-110 || E -.733 || B -1.296 || C -37.88
|-
| Q-111 || E -1.06 || B -.563 || C +1.296
|-
| + side OP device || E 120 mv || B +.728 || C +38.09
|-
| - Side OP Device || E 140 mv || B -.733 || C -37.89
|-

[h=Setting the bias ]%2[/h]
From [Post #2508].

Setting the bias is easy...turn the pot so the DC you measure across both the driver emitter resistors is at a minimum. Connect the driver board to the outputs, then if you can, slowly apply power to the whole shooting match, monitoring current draw, anything more than a few 10's of milliamps is bad, you'd want to turn everything off and check your wiring......If you can't do it slowly, then just turn it on...from a distance?

Anyway once you have all the parts connected, power applied and no smoke issuing from anything, attach a meter to measure the voltage across one of the 0r68 output emitter resistors (say Re1), and another to measure the output voltage relative to ground, and if you have 3 meters (and who doesn't?) attach another one to measure the total voltage across the driver emitter resistors r127/128...

Slowly turn the bias pot to increase the output idle current, you can also measure this as the voltage across the 25ohm driver emitter resistors, nothing happens at first, basically until the board output voltage reaches about 1.2v across r127 & 128, then the outputs start to turn on and you will read a voltage across Re1...keep increasing until you have about 0.2 volts across Re1...with 3 output pairs this corresponds to ~1A total idle current, you'd probably want to allow this to sit for a little while check temps, make sure the voltages across all the output emitter resistors are nearly the same, any big discrepancy should be investigated. Once you are satisfied that everything is OK, start increasing the bias until you see ~0.4v on Re1...as the temperatures on the sinks change this is going to move around, once it seems to have settled, adjust the other pot to reduce the DC offset to the lowest possible value (<50mv was easy to achive on all the boards I've made). This will wander as the temps change, so repeat the process a few times over the course of an hour or two...

(NOTE- .4v across Re1 is only valid with .68 emitter resistors, do not attempt this with lower value resistors-- compute the class A power you will get separately. With 4 output device pairs and .499 ohm emitter resistors 50 W class A into 8 ohms is about .22 V across the emitter resistors, into 4 its about .32V).

The bias resistors (R144, R126) are setup such that:
Lower R=Higher Bias; Higher R=Lower bias.

See [Post #2527-28], or, according to [Post #2531]:

If you are not sure about the bias and need to begin at the low bias point and are not sure which way to turn the trimpot to reduce bias, here is a foolproof trick that has helped me over the years.

Disconnect the driver's emitter to the output's base (saving the outputs from being driven). Then measure the voltage across the B and E of the driver, less than 0.6vdc and you are ok to go ahead with initial biasing, (This is
class AB territory) above 0.6vdc and you are in high bias/ Class-A territory and you need to lower the bias to 0.5 before you connect the driver to the output stage. This method is a quick test of verification and not extremey
accurate but very practical if you are not sure which way the trimpot should be.

Class-AB amps need to have about 0.50 to 0.55 vdc across the BE of the output devices as a quick test. Another quick test I do it use a 100-200 watts lamp in series with the amplifier, then turn the amp on and set the
trimpots such that the lamp goes down to a dull amber... this is low bias setting. Then remove the lamp and set the bias correctly using the voltage readings off the OP device's emitter resistors as others have explained.
Setting your particular bias to a certain number of Class A watts is described in [Post #3094] and [Post #3106]