My 50 Watt RMS design using Randy Slone's formulas - diyAudio
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Old 10th January 2007, 07:28 PM   #1
corrieb is offline corrieb  South Africa
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Default My 50 Watt RMS design using Randy Slone's formulas

Hi All,

Two years ago my wife bought me a set of KEF Q4’s for my birthday, and I have been wanting to build my self a decent stereo amplifier my speakers.

A couple of months ago I bought my self G. Randy Slone’s book (High Power Audio Amplifier Construction Manual). The information in this book thought me a lot about designing amplifiers with all the formulas. What I don’t like about the book is that there are uncertain factors like the gm of the input stage or the RC (stability components on the feedback rail).

Anyway I used the formulas to design an amplifier and the schematic attached is the result. I did do intensive simulation using Electronics Worksbench’s MultiSIM version 8 and the amplifier seems to be stable. The current theoretical specifications according to the application are as follows.

Input Impedance: 22K
Input Sensitivity: 1v RMS
Output Power: 60 Watts RMS into 6 Ohm (19.2v RMS)
Bandwidth: 2 Hz – 160 kHz (-3 dB)
Distortion: 2nd Harmonic (380 uV; 0.0019% @ 6 Ohms full drive @ 20 kHz)
3rd Harmonic (4.1 mV; 0.021% @ 6 Ohms full drive @ 20 kHz)

If some of you experts out there can please comment on my design I will really appreciate very much. Any feedback whether good or bad is most welcome as well as any pointers for the do’s and don’ts will be appreciated.

Thanks,
CorrieB
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Old 12th January 2007, 10:01 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
transistor choice seems too varied.
All the BC 550/560 could be the same grade, use c grade rather than a or b, unless Sloan has given a reason for low gain types.

Some of the other types could also be made similar or even the same but I don't have data on them.

Are you making the PCB yourself?
Then include base resistors on all the LTP inputs (4 off).

R19 C3 has the correct RC time constant for 1uS. but the small resistor value (low noise) and large capacitor value (poor high frequency) do not get optimum. You are trying to attenuate all above audio frequencies, including RF. a small cap does this better and since it's effect is virtually out of the audio band a high audio quality cap is not so important. an NP0 (=C0G) ceramic might be ok or a 680pF MKS, MKT, PES, PP etc would all do the job. If you adopt a bulky cap then allow space for a tiny RF cap in parallel. Be prepared to experiment between 0.5uS and 1.5uS to suit your speakers/ears/sources. Some even suggest pushing this to 0.3uS.
The DC block is right on the limit of my usual recomendation (80 to 100mS). But the NFB DC block is set to 264mS. This is lower than half an octave below the input DC blocking filter so ok, but the PSU filter needs to be a further half octave lower again and that target becomes nearly impossible to meet for 6ohms if you aim for 264*1.41=372mS. You will require +-62mF of smoothing capacitance per channel (a bit bulky and expensive). It will give extended bass but might interfere with mid and treble quality.
The FB point is shown between the emitter resistors, move the tapping point to the link between r24 and r25.
Are you planning to mount the Thiel network on the speakers terminals? If not then you MUST ensure that the ground return does not contaminate the other PCB grounding and cause oscillation.
Add a cap in parallel to r17 in case you need to adjust stability and/or output overshoot into slightly capacitive loading.
Consider adding a pair of series diodes (1.4V) across C7 to prevent reverse DC across the cap in event that the output offset goes high on fuse blowing or transistor failure. The cap voltage rating must be either rail voltage or add another pair of diodes or single Zener to protect the caps for offset the other way.
C9 may sound better if it is replaced with a polypropylene (or other high audio quality cap). But probably a lot smaller, try 1uF.
Leave space for on board decoupling caps from rail to power ground. Both electrolytic (100uF to 1mF) AND Hi-K ceramic 10nF to 100nF.

Make sure your audio ground and power ground are kept completely separate on the PCB.
Some of your grounds carry only DC currents and some carry low value AC currents and some medium value AC currents. All these are best returned to a PCB signal star ground. The high current grounds and those that see half wave AC currents must be combined separately from the clean ground.

That's enough to think about for now.
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Old 12th January 2007, 05:12 PM   #3
corrieb is offline corrieb  South Africa
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AndrewT,


Thanks for all the valuable information!

Do you have some kind of base line theory/recommendation that I might use for designing audio amplifiers? I am not an expert when it comes to electronics but I really want to learn!!! Where or with what kind of design/kit do you recommend I begin with?


Thanks
CorrieB
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Old 12th January 2007, 05:54 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
you already have Sloan (I've not read his).
add
John Linsley Hood, the Art of Linear Electronics.
Doug Self, Audio Power Amplifier, and/or web site.
Leach low tim web site.
Elliot Sound Products web site.
Nelson Pass web site.
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Old 12th January 2007, 08:03 PM   #5
corrieb is offline corrieb  South Africa
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Hi,

thank you for all the good reference information



CorrieB
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Old 13th January 2007, 03:28 AM   #6
thanh is offline thanh  Viet Nam
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AndrewT!
Quote:
sloan
Do you mean "Slone" ?
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Old 13th January 2007, 08:20 AM   #7
AndrewT is offline AndrewT  Scotland
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slone/sloan probably, but as I said I have not read it.
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Old 13th January 2007, 10:31 AM   #8
jaycee is offline jaycee  United Kingdom
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The schematic looks good to me. The only thing I would really change is the multitude of transistors being used.

For current sources, mirrors, i would just use BC546B/556B in place of 2SA733/2SC945.

The VBE multiplier (Q15) I would probably use MJE340 or BD139 here so that it can be easily attached to the heatsink for thermal tracking.

I would not use MJE340/350 as drivers - I would instead use drivers such as MJE15034/5 or MJE15032/3.

The use of medium power 2SA916/2SC1941 as VAS buffers (Q11/Q12) strikes me as pointless - again BC546/556 would do here. The NEC transistors look quite good for the VAS though, as they are fairly fast and have a low Cob. If sourcing them is an issue, BD139/140 would do.

edit: I say all this assuming it is easier to get european devices where you are
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Old 13th January 2007, 10:37 AM   #9
corrieb is offline corrieb  South Africa
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Thanks for the recommendations, for the ouput transistors what will you recommend I rather use the current once in the circuit or perhaps the MJ15003/4 pair?
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Old 13th January 2007, 10:42 AM   #10
AndrewT is offline AndrewT  Scotland
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Hi,
there is little similar between MJL1302/3281 and MJ15003/4 other than ONsemi!
That is an enormous change from the published design.
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