Built my first amp ! (Randy Slone Design 4) But it is humming...

[jpnascim]

Well, after some time I’ve managed to construct my first audio amplifier! It is an amplifier from Randy G. Slone. For those familiar with the book it is the design 4 contained in the book High Power Audio Amplifier Construction Manual. I saw many threads here in this forum regarding the OPTIMOS which is the flagship of Randy Slone’s designs but I haven’t seen many about this design I built. So I thought it would be nice to share my experience.
Here are the characteristics:

- BJT differential input (2N5551 and 2N5401)
- Darlignton VAS
- BJT OPS output
- It has OPS output protection (V-I Limiter)
- 300 VA E-I transformer (couldn’t get a hand on a toroidal)
- 2 x 10.000 uF capacitors in the PSU
- Volume adjustment and no tone controls
- Rated at 100 W rms (8 ohm).

I’ve made some parts modifications:

- Output transistors are MJL1302/MJL3281 from On instead of the Toshibas which are hard to find

- VAS and some other transistors are the KSA1220 and KSA 2690 form Fairchild instead of the 2SB649/2SD669 from Hitachi which are rare and not manufactured anymore as far as I know.

Unfortunately I don’t have a digital cam to send some pictures but I’ll try…


I haven’t had the opportunity to listen to many high end amplifiers in my life but I can say that this amplifier is excellent! The sound is pure, not exaggerated, precise and very pleasant to listen to (even though my pair of speakers is not Hi-Fi)

I feel great ! I thought I wouldn’t manage to do it since I am a newbie. It worked right at the first time (lucky me !) but I still have one minor problem that I ask you guys to help me: it is humming. From what I’ve read the problem has to do with improper grounding.

Well, attached is the wiring diagram of my amp. I hope it is clear for everyone. One problem that I’ve identified after reading a lot of star grounding posts here is that the signal input ground (shield of the cable) is connected to the rail decoupling and feedback ground in the PCB board (left and right pwr ground – see figure). So I guess I am mixing dirty and clean grounds. Besides that, anything wrong ??

Thank you,

João Pedro

[mwmkravchenko]

Congrats sir now you are an official DIY'er

Mark

[Tube_Dude]

olá João Pedro

Are the RCA inputs, isolated from the chassis?

[carlosfm]

Oi João Pedro,

Instead of placing the power ground near the PSU, you could place it right between the two amp boards, with thick and short wires from the two channels to the star grounds.
The distance from the star ground to the PSU is not so important.
The distance between the two channels and the star ground IS important.

I don't know if your sketch represents your layout faithfully in this matter, though...

[jpnascim]

Originally posted by Tube_Dude
olá João Pedro

Olá Jorge !

Are the RCA inputs, isolated from the chassis?

Well, I forgot to mention but the amp isn't inside a chassis
This is because still I need to find one (which is the hardest thing to obtain here in Brazil). The shield of the cable for each channel is conected to its respective RCA connector and then it runs near to the PCB where the input conductor is soldered. There, I join both shields and a wire, forming a "T", this wire runs to the star ground. This was the only improvement I could get to reduce hum.

I would like to know if I am asking for too much when I want absolute silence from an amplifier that is sitting on a piece of wood with no chassis to work as a shield.

Thank you,

João Pedro

[jpnascim]

Originally posted by carlosfm
Oi João Pedro,

Instead of placing the power ground near the PSU, you could place it right between the two amp boards, with thick and short wires from the two channels to the star grounds.
The distance from the star ground to the PSU is not so important.
The distance between the two channels and the star ground IS important.

I don't know if your sketch represents your layout faithfully in this matter, though...

Oi Carlos,

The sketch represents pretty closely my current wiring layout (you can even assume wiring length scale ) There are 2 thick and short wires running from the PSU to Star-Ground and 2 thinner but much longer wires running frm each PCB dirty ground (LEFT and RIGHT PWR GROUND) to the Star ground.

My transformer is making a little bit of noise (eddy currents??) and maybe that is passing as considerable noise to the Star Ground via the 2 short thick wires... Is it possible ?

I've made some progress. Yesterday, I unsoldered the input connector from the PCB (to examine how much hum is coming from the AMP Channels PCB). I noticed significant reduction (but it wasn't 100 % silent). Maybe the chassis will solve most problems (don't know) since my RCA connectors, input cable and potentiometer may be getting a lot of noise from the environment. Just a theory....maybe I am wrong.

Originally posted by carlosfm

The distance from the star ground to the PSU is not so important.
The distance between the two channels and the star ground IS important.

Thanks for the suggestion ! So, do you think it is a good idea to user shorter and thicker wires for the AMP Channels Power Grounds (LEFT and RIGHT PWR GROUND) and use longer wire between PSU ground and Star ground ?

Thank you very much,

João Pedro

[carlosfm]

Originally posted by jpnascim
Thanks for the suggestion ! So, do you think it is a good idea to user shorter and thicker wires for the AMP Channels Power Grounds (LEFT and RIGHT PWR GROUND) and use longer wire between PSU ground and Star ground ?

The shorter the wires between the two channels and the star ground the better.
Also use thick wires.
You need a low impedance path to ground.

Regarding the PSU, always use thick wires, but the location is not so critical. It could even be external.

[Tube_Dude]

João Pedro

1- Disconnect the two wires that come to the amp board, from the star ground.

2 - Connect a thick wire between the two points in the two boards , were previously have been connected the two wires that come from the star ground.

3- From the middle point of this wire (between the two boards ) connect a thick wire to the star ground.

Thats all

Um abraço.

[jpnascim]

Yesterday I've made some modifications in the STAR GROUND wiring and I've managed to reduce hum almost completely (when room is completely silent you can hear just a little) and I consider it very satisfiable. When I put the amplifier inside a chassis I think it will get even better !

Attached is my new wiring diagram that reflects the changes I've made according to the sugestions of Carlos and Jorge, which helped a lot !

To summarize I will give the recipe:

- 1 long and thick wire running from PSU board to the STAR GROUND;

- Separate wires from each speaker return grounds (thick wires too) to the STAR GROUND;

- Short and thick wires running from each AMP channel to the STAR GROUND. This was one point that really made the difference. Although the recomendation was to join them together and then run a single wire to STAR GROUND I got better results with two separate thick wires goind to STAR GROUND;

- I've connected the shields from each input wire together with a wire that is thinner than the previous ones. From the middle point, I've run a thin single wire to the STAR GROUND. This was the other point that made a big difference.

It is amazing how small details really make a huge difference in hum reduction. Good grounding is surely an art. The benefits are worth it !

Thank you Jorge and Carlos

Abraços aos 2

[jpnascim]

Here it is !