On 05.01.2018 at 6:24 PM, sbelyo said:

2. The highest hfe is the BC546\556 C, this only comes with formed leads.  Will I need to straighten them out, and if so I'm guessing there's no issue with that?

No, formed leads are what fits best for this board (though looking at the PCB you might think that lead spacing is for straight leads. No, it for formed.

cool, I've yet to sort the formed lead devices

To be honest, modern BC546/556 from ON are measuring like crap. Last lots I bought (genuine of course) measured Hfe 250-280 @ 10uA base current, some hardly reaching 300.

Try to get from AMB his NOS stock from Fairchild. As I remember they measured 300-330 and up consistently. If I recall correctly, BJT transistor leads from amb didn't magnetize as well.

Now I feel sorry that I didn't have time to order and have them shipped from him for my CFA amp which is in progress.

Edited March 5, 20188 yr by Helium

Also, I wonder how the KSA992FTA/KSC1845FTA would work in this? F grade is supposed to have hFe 300-600, and Mouser has them.

EDIT: pinout is different

Edited March 6, 20188 yr by Pars

I buy all my sand on Mouser and They say that for example On/Fairchild BC556BTA grade has a measure of HFE/450.

I have around 200 eas on my warehouse I need some time for measure some units but I guess that they don't lie... or this I would like to believe. 

Edit:  I'm seeing that I have some KSA992FTA/KSC1845FTA but yes, Pars is right; It's a shame they have different pinout. 

 

Edited March 6, 20188 yr by jose

I measured all my BC556/546 BTA stock: hfe from 299 to 416 but the most abundant value is in the fork from 320 to 380. I don´t know whether to do it with these values.

I have 100 BC546CTA bought in Mouser too and today I found a guy who bought him another 100 units of the BC556C ... I hope to be lucky.

Yesterday I tested my pcb's and they seem to work correctly. Offset on un/bal output is 0mVolt (with servo).

With buffer I measure an offset of 8mVolt on the output in each one. It's right? I'm using OP27.

Should be the same... 0mV


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On ‎23‎/‎01‎/‎2017 at 4:32 PM, kevin gilmore said:

yep, gets the output impedance down to say 5 ohms. then you can add some resistance to match the cables you intend on using, +45 or +70 ohms

Do I need use any resistance on the Buffer output? 

 

If you are driving 20ft or more cables, 50 ohms is a good match for the cables. For headphones no resistance necessary

13 hours ago, jose said:

Yesterday I tested my pcb's and they seem to work correctly. Offset on un/bal output is 0mVolt (with servo).

With buffer I measure an offset of 8mVolt on the output in each one. It's right? I'm using OP27.
 

Sounds like a transistor matching issue. What's the offset without the opamp?

also you might want to try the ubal board without a buffer and see if it makes a difference. If you have an amp with 50k input impedance then it might not be a major issue.

There is virtually no difference between using the OPA or not (I understand that I should not either, right?  Opa here is not working as a servo). The four buffer have more or less the same offset (8mvolt)

All fets are matching around 530 ~ 540hfe and 540 ~ 550hfe (P to P and N to N) Is a closer match necessary?

If so, which would be the most critical?

On ‎26‎/‎03‎/‎2018 at 11:18 PM, cspirou said:

... if you have an amp with 50k input impedance then it might not be a major issue.

CFA2 only has 10K input impedance so if I want to use buffers I´d have to add a 40k resistor on the buffer output. 

 

It's the complex impedance of the cables that can cause oscillations if driven from a zero impedance output.

input impedance of the amp does not matter.

this is why a buf634 can oscillate and it always has a gain less than 1

I´m using the un/bal boards like preamp with my CFA balanced. 

Input/selector -> 10k pot -> un/bal -> CFA  

It work perfectly so I´m very happy. Now I can start my preamp chassis.  

With the buffers I need to think what can I do because I don't feel comfortable using it with Offset. Obviously the easiest thing would be to add capacitors in the output, but clearly that is not the goal. I will try to make several set of macheted transistors to see if I can get one that is below the 8mVolt that I currently have. 

 

On ‎03‎/‎02‎/‎2017 at 4:03 PM, sbelyo said:

If I wanted to make a preamp with these boards can I omit the attenuator and wire it like this?

For example:

 

I´m working on my KG Preamp unbal/bal. I was setting up my preamp in the following way:

Input- Volume-Unbal/Bal-Buffer-output.

Dr Gilmore recommended me to change the order and put the volume between the Unbal / Bal and the Buffers, so now is:

Input-Unbal/Bal-Volume-Unbal/Bal-output. 

 

good to know...  Was this to reduce the offset?

59 minutes ago, sbelyo said:

good to know...  Was this to reduce the offset?

Nop... Apparently the right place for the Unbal/bal is before the volume pot. I guess it worked better with a stable input voltage. Maybe Dr. Gilmore can explain it to us.

Offset is a different problem. We are working on this. Apparently it is easy to solve without changing our pcb's. I just fixed one of mine, but surely there will be an improved PCB.

Basically we already know how to fix it. Modifying the 250 Ohm resistors... at least in my case.

I need to modify the rest of my buffers to compensate the offset. Let me experiment a little.  

5 hours ago, jose said:

 

Dr Gilmore recommended me to change the order and put the volume between the Unbal / Bal and the Buffers, so now is:

Input-Unbal/Bal-Volume-Unbal/Bal-output. 

 

Don't you mean Input-Ubal/Bal-Volume-Buffers-output?

I mentioned something like this earlier but his response was to put it before the Ubal/Bal board

https://www.head-case.org/forums/topic/12733-balanced-to-unbalanced-board/?do=findComment&comment=753597

However that's because I have the SMD boards. The through-hole boards have a servo for the buffer so is probably different.

The VAS before attenuation is good for low noise since you aren't amplifying Johnson noise. Downside is the Ubal/Bal stage possibly clipping because it is always getting a full signal. This should not be an issue though because the gain is low and the rails are +/-20V

5 hours ago, cspirou said:

Don't you mean Input-Ubal/Bal-Volume-Buffers-output?

I mentioned something like this earlier but his response was to put it before the Ubal/Bal board

https://www.head-case.org/forums/topic/12733-balanced-to-unbalanced-board/?do=findComment&comment=753597

However that's because I have the SMD boards. The through-hole boards have a servo for the buffer so is probably different.

The VAS before attenuation is good for low noise since you aren't amplifying Johnson noise. Downside is the Ubal/Bal stage possibly clipping because it is always getting a full signal. This should not be an issue though because the gain is low and the rails are +/-20V

Yes, yes, I followed the recommendation of that post but now I have changed the volumen, putting it between the unbal/bal and buffers. It is also true that you use an attenuator (relay or digital I´m not sure) and I am using a volume pot.

18 hours ago, jose said:

Nop... Apparently the right place for the Unbal/bal is before the volume pot. I guess it worked better with a stable input voltage. Maybe Dr. Gilmore can explain it to us.

Offset is a different problem. We are working on this. Apparently it is easy to solve without changing our pcb's. I just fixed one of mine, but surely there will be an improved PCB.

Basically we already know how to fix it. Modifying the 250 Ohm resistors... at least in my case.

I need to modify the rest of my buffers to compensate the offset. Let me experiment a little.  

ok, I'll hold tight to see what the fix is.  I'm still matching devices anyway

Not as elegant as a servo but works

where's the fix?  I missed it

1 hour ago, sbelyo said:

where's the fix?  I missed it

If you have not noticed, it's been better than I thought. 

 OK, use a 500 Ohm trimpot on 250R place (Rail +) and maybe change 10K resistor on the OPA (- Output to Ground).

However, Dr Gilmore is still working on a solution (Thanks Dr. Gilmore for you patience)

oh I see it now...

I'll keep watching to see what the final fix is