(AK) classA HA/Preamplifier.

[Deleted]

I'll give them a while to settle in, reset the bias (I'm running 400mA) and re-balance the front end.

Hi Jon
Let us know if the bulk foil resistors have settled down when you have a few more hours on them.
If I were to up the bias to 400mA it would get very warm in it's 1U rack case.
I would be worried about reduced life expectancy of the electrolytic capacitors due to the extra heat.
Perhaps any more than 200mA per channel is overkill ? Can you hear any further improvement from going to 400 mA bias ?
TBH, I am concerned that you may be pushing the 2SA1930 and 2SC5171 too far with their power dissipation, if you are running at 400mA bias per output device.
You would need to check their data sheets ?

Do you need reading glasses in the cockpit ? I know you don't need glasses normally though.

Kind Regards
Alex.

[jonclancy]

That'll teach me for posting without checking!! Got an excuse as I'm under the weather at the moment.

I am running 400mV/25 = 160mA bias.

Doh!! I would be wondering what the burning smell was if I was running 400mA bias! Thanks for the speedy interjection, though Alex.

Yep - long distance vision is OK, but as many of us know, once you get into your forties... Readers required. Might even be on the way to varifocals at some stage. Jeez!

[Deleted]

That'll teach me for posting without checking!! Got an excuse as I'm under the weather at the moment.

Hi Jon
Sorry to hear that. At least you didn't get the Aussie version when you were out here recently !
Kind Regards
Alex

[drymdrum]

Hi SandyK,
I'm currently building two spare boards for the AK Class A.
BC549C and BC550C have virtually the same specs'......................you recommend T7 BC549C to have hfe of 500.

I have only 1 BC549C with that hfe all others are around 300.

I have many BC550C at 600 plus so could match T7 for both boards.

What are the reasons for using the BC549 with its lower max  vceo/vcbo against the BC550?

Regards,

Alan 

[Deleted]

Mar 5, 2014 22:09:11 GMT drymdrum said:

Hi SandyK,
I'm currently building two spare boards for the AK Class A.
BC549C and BC550C have virtually the same specs'......................you recommend T7 BC549C to have hfe of 500.

I have only 1 BC549C with that hfe all others are around 300.

I have many BC550C at 600 plus so could match T7 for both boards.

What are the reasons for using the BC549 with its lower max  vceo/vcbo against the BC550?

Regards,

Alan 

Hi Alan

 The BC549C has only about 2.7V across it. It is also specified for low noise, and may be quieter than the BC550c.

This would depend on the actual manufacturer, with later data sheets from ON and Philips showing the only difference as voltage ratings in the case of Philips devices, and the same voltage ratings given by ON.

I suspect that the ON data sheet may be incorrect in specifying 45V for the BC549c as well though.(?)

I would use 2 BC550c with HFE as close to 600 as possible, to avoid the possibility of instability in your layout.

Kind Regards

Alex 

[drymdrum]

insert quote here

"In your case I would use 2 BC550C with as close to 600HFE as possible, just to make sure there are no instability prob;ems."





Alex I like your style................nothing like rubbing in the non SC layout..............however I have an idea on the instability and I'll make the "Layout" alteration tomorrow....later today.......that is.
I'll just use the BC550s thanks for confirming my thoughts
I've already made it on the new boards a very minor movement. Just had to buy a few caps and resistors they arrive today from RS made my order to them at 7.30 last night about five hours ago.
Was close to our local Maplin shop yesterday so went in for the resistors I'm short of. That was a shocker buy ten for a discount dropped the price to 30p each that'd would have cost me £19.50
RS price for 65 = £2.80 
Regards,
Alan

 

[Deleted]

Hi Alan

No, not trying to rub it in. I had instability signs with the SC original layout when using much higher HFE devices there.

I was pleasantly surprised that we didn't have problems with Will's slightly different layout after the SC comments about their PCB layout.

 In one later design they even ran twisted supply rail wires straight up the heavy central earth track underneath the PCB using cable ties and then fanned out + and - to respective sides.

That's why I tried to compromise by suggesting 450 to 550 HFE. I have heaps of them well above that HFE and would have loved to have been able to use them due to lower loading on the preceding stage.

Regards

Alex

[jc]

Alan, we're still in awe of your routed board designs!


I'm yet to get as far with ready-made boards. Hopefully soon and then Alex will be fielding my problems too.

[drymdrum]

Nice to have chit chat on here again with you both.
Let's wait and see what my mod to the board with  dratted whine achieves later today.
I'm hopeful that the minor alteration will cure it.

I'll take before and after shots.

Regards

Alan 

[Will]

Mar 6, 2014 1:22:33 GMT @sandyk said:

I was pleasantly surprised that we didn't have problems with Will's slightly different layout after the SC comments about their PCB layout.

Good to see I got something right with that PCB, even if by luck than design!!

[Deleted]

Mar 7, 2014 9:31:33 GMT Will said:

Mar 6, 2014 1:22:33 GMT @sandyk said:

I was pleasantly surprised that we didn't have problems with Will's slightly different layout after the SC comments about their PCB layout.

Good to see I got something right with that PCB, even if by luck than design!!

Hi Will

 Don't forget that due to JLHs for the front end with their very low impedance vs. fuses,we could have run into niggling problems.

Yours is also dual layer with vias between sides. It's quite different to the original due to that alone.

 Even with the original S.C. PCB  I learned the hard way not to use BC 549C with HFE of 800 .

 I even tried fitting ferrite beads in a couple of locations.

Kind Regards

Alex 

[drymdrum]

Hi Alex,
we return to the whine tonight having made the minor mods which have made no bloody difference with regard to the temperature related whine.

Question............why no whine when listening on headphones yet whines through speakers?

Below is retaken voltages all are similar to yours including the c/e voltage on T8 BC547 which is 1.627v, yes I was taking the reading from the wrong place

I've two replacement boards fully built but untested, so it's out with the faulty board and in with new tonight, let's see where I get to before bed.
Regards 
Alan

  

[drymdrum]

[drymdrum]

New Boards

[Deleted]

"I've two replacement boards fully built but untested, so it's out with the faulty board and in with new tonight, let's see where I get to before bed."

Hi Alan

 I have my fingers crossed for you !

Your particular problem is unique, and I have never struck anything like it before. None of the other constructors reported similar either,

 so I am at a loss to explain why at the moment. I am wondering about grounding issues though.

Are the centre taps of both primary windings well earthed to "chassis" at the inputs to the PSU PCBs ?

There should be no connection to IEC mains earth in the preamp itself. The earth to the preamp comes via the 2 x 10 ohm "earth lift" resistors in the P.A.

Regards

Alex

[drymdrum]

Alex,
I think you may have it by the short and curlys 
before I move forward I'll solder in a link to chassis and 0V on the 20 volt inputs for both PSU boards.
IEC earth will be at PA as before.
How did I miss that one, have to check my sons build also.

Regards,

Alan 

[Deleted]

Hi Alan

I have attached a photo of my preamp. It no longer uses the relay PCB at the rear, as the 15W Class A already has a time delay at switch on in the Loudspeaker Protector PCB. The thin red and white output cables were also replaced by heavier duty screened cable.

You may also see that the CT's of the transformer primary windings are also connected directly to chassis at the incoming XLR sockets.

 Not shown here is an additional earth wire from the furthest away module to it's JLH. This helps to equalise earth impedances of both modules.

 You could of course use a heavier gauge earth wire to the more distant module. I was fortunate in having vacant earth terminals on both PCBs.

Surprisingly, the extra earth wire did make a small audible improvement.

Kind Regards

Alex

[jc]

Taking notes ....

[drymdrum]

Hi Alex ,
thanks for the pointers on earthing the AKSCHA/Pre, although I should have been onto that one when I re-cased the original preamp (my own),
with the second build for my son he reports that earthing is as should be.

Last night saw the two new modules tested............first module has problems so I'll get back to that one soon.

However second module is sitting in preamp all wired up and sounding great, I set the bias at 150mv and LTP balance as close to zero as I could,
hovering around zero that is. Offset was  0.7mv without the OPA134 and -0.5mv with it in.
Have to say it sounds as good if not better......expectation bias I suppose......but it is to my old ears.
Second module did have a fault but minor as it turned out.........I'd -20v okay on the input post but no voltage 30mm down the -20v track,
turns out I'd sliced through the track sufficient to impede the flow but requiring the use of a 10x loupe to see.

Now let it soak test for a few days and hope as the case comes up to running temp the new module behaves itself.
Perhaps I should set bias at 200mv same as it's partner...............just done that.

When the new module is proved as a non whiner the old one goes back in just in-case it was the  earthing at fault although
I doubt it as it whined in the first case I built for it and it's case earthing was done properly......although everything else was a mess.
So here's hoping,
regards,
Alan

[drymdrum]

Two hours of play temperature reasonable but whine now on both channels!!!!
The saga continues.
Alan

[Deleted]

Mar 13, 2014 19:21:26 GMT drymdrum said:

Two hours of play temperature reasonable but whine now on both channels!!!!
The saga continues.
Alan

Bummer !

Are you able to look at the supply rails with a C.R.O , or  see if you can measure any A.C. on the supply rails with a DMM ?

Is the whine there at minimum volume control setting ?

Are both modules still quiet when being used solely as a H.A.  ?

Perhaps a series of close up internal photos showing all wiring etc. ?

 Especially a good close up of the PCB itself. Do you have a 100uF and 100nF in parallel per supply rail, at 2 different locations, in addition to the bypass caps for the offset corrector ?

email those.

[drymdrum]

Hi Alex,
bummer.....yes but we'll get it fixed sometime.

 Are you able to look at the supply rails with a C.R.O?................................No

or see if you can measure any A.C. on the supply rails with a DMM ?
...........................Yes can measure at JLH out

Is the whine there at minimum volume control setting ?
.....................................Yes.............Volco has no control on whine.
                                                                                                                                          frq range is mid rather than high/low

Are both modules still quiet when being used solely as a H.A. ?
..............................Yes

Perhaps a series of close up internal photos showing all wiring etc. ?............................I'll look at my stock photos.

Especially a good close up of the PCB itself...............Yes will do

In process of re-jigging output wiring
 with screened cable
boards are out switch and volco board also partly disassembled to let me get at the bottom contacts. 
I'll replace the input/output  cable to the back of the switch board with screened while I'm at it.
 
So I'll finish that get the second new module working throw it all back up and go from there with two new amp modules.


Do you have a 100uF and 100nF in parallel per supply rail,
at 2 different locations, in addition to the bypass caps for the offset corrector ?
..........Yes
email those.



Regards,
Alan

[drymdrum]

or see if you can measure any A.C. on the supply rails with a DMM ?

...........................Yes can measure at JLH out
#


Here are the voltages showing unloaded as boards are out 

Left side PSU out at JLH..................+ = 0.157va/c.......................... +20.32vDC
Left side PSU out at JLH................. -  = 0.000va/c.......................... -20.39vDC

Right side PSU out at JLH............... + = 0.113va/c.......................... +20.33vDC
Right side PSU out at JLH............... - = 0.000va/c........................... -20.51vDC
Regards,
Alan

[Deleted]

Mar 13, 2014 23:31:08 GMT drymdrum said:

or see if you can measure any A.C. on the supply rails with a DMM ?

...........................Yes can measure at JLH out
#


Here are the voltages showing unloaded as boards are out 

Left side PSU out at JLH..................+ = 0.157va/c.......................... +20.32vDC
Left side PSU out at JLH................. -  = 0.000va/c.......................... -20.39vDC

Right side PSU out at JLH............... + = 0.113va/c.......................... +20.33vDC
Right side PSU out at JLH............... - = 0.000va/c........................... -20.51vDC
Regards,
Alan

Alan

 Interesting results. Can you try loading those +VE rail JLHs with a resistor to see if it drops back down again, or perhaps even with the amplifier working ?

Regards

Alex

[Deleted]

Just as the 15W Class A amplifier PCB suppled to RG members had a PCB track error, which saw the Earth of the output connected to the 10 ohm earth lift resistor location, the Class A HA/Preamp also has a similar PCB error. However, as it uses a link instead of the 10 ohm resistor of the power amplifier, it will operate normally. If anybody has their Class A HA/Preamp out for checking etc. I would recommend installing a small link under the PCB  between the earth of the output and the adjacent earth terminal block.

 There may not be any noticeable audible improvement, but the distortion measurements  are likely to be a little better after this minor correction.

Alex