Gyrator loaded Son of ZV9/F3

HI Bigun,
Sorry it took so long to answer, I work on 5 projects at a time.
Today I lowered R13 from 4.7K to 1.8K so I could try it with headphones.

How does it sound? Nice, no complaints, it does seem to wake up the sound,
kinda like the difference between an F5 and ACA. more forward and detailed.

today's test setup is with a ACA made with To220 mosfets and running on 16V into Ortho's
Yummy.

933961_HDR.jpg
 
Hi,

I have run the amp today. I does play music, but I have very high level of hiss/somewhat white noise.
I have had issues with noise in a different amp before and it was due to some zeners from China.
Here I am pretty sure my jfets are from a trusted source (RS Components).
I am not absolutely sure where the BC550 comes from.

In my version I lowered R13 to 3K3 to lower the gain. Also my BF862 Ids was some 17mA as far as I remember but I did not increase the source resistor. Could it be the problem?

I should add that I am running it form a printer power supply just as Juma with a capacitance multiplier (I tried to bypass the multiplier to see whetehr it is injecting noise, but the hiss was not gone but I obviously got hum. I also tried a regulated linear PS and the effect was similar. So it seems the noise is in the preamp).

The volume control is 10k.


Here are my values (in reference points provided by Juma in post #54)
1. across the R2 (4R7) - should be about 50-60mV I see 60mV
2. across the R8 (47R) - should be about 500-600mV I see 600mV
3. between the source of Q6 (BS170) and the GND - should be about 27V I see 27V
4. between the base of Q4 (bc547c or bc550c ) and the GND - should be about 13.5V, I see 13V
5. the Vgs of Q6 - should be about 2.3 V I see 2.6V
6. the Vbe of Q4 - should be 650-700 mV I see 600mV
7. across R12 (680R) - should be about 7.5V, I see 9.7V
8. across R1 (10R) - should be 100-120 mV, I see 140mV

If anybody has any idea let me know. I would prefer not to desolder active components one by one and replace them blindly.

Edit: As a temp solution I lowered the gain to 5.7dB (the hiss is still there but less annoying). In fact I can use the volume control better now. Can I reduce the gain even further without ill effects? My normal preamp is B1, so I do not need gain at all.
 

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Just another Moderator
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Gregje, you have some very large resistors there on the input. 1M ohm and 2.2M Ohm, also the 2.2 seems to be a 5% type so I would think it would not be a "low noise" one and probably carbon comp.

Any reason you are using such large values here? I looked at Juma's original circuit and it has much more "normal" looking values.

The higher the resistance the higher the Johnson noise, this is especially so for input and feedback resistors....

Sorry if I'm spouting nonsense, my electronics is not real crash hot, but that seemed a little bit odd to be.

Tony.
 
Hi Winternute,

You are probably referring to the original schematics at the beginning of this thread.
The final version is in posts #24,#37 and the attachment of my post above. It says 1M and 2.2M. Having said that I agree with your comment that the higher the resistance the higher the noise. On the other hand reading Juma's posts (in general not only here) I would assume his circuit works without fancy parts.

I need to do more thinking and experimenting...
 
Tried replacing bc550C with another npn and bf862 with 2sk170 (all my bf's are from the same batch and both channels hiss so there is little sense trying a different one). No change in both cases.
All my bs170 are from the same batch again, so need a different batch or an equivalent.
And then to passive elements...
 
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Gregje: Before blindly changing out all components, if you do not have a scope, beg, borrow or steal one (well...maybe buy instead of steal) and check the output to identify this nasty noise you do not like. Then work your way back one step at a time with the probe until it goes away (or work the other direction) to identify the place the noise starts. It might even be at the 40VDC input, or an input connection, etc. In your case I do not think you need to apply a signal generator to the input, but for some issues you do.
I am using the first version of this preamp and it sounds fantastic, and I plan to build the latest version soon. I think it will be well worth your effort to get this working.
 
Hi Propitious,

Our posts have crossed.

Thank's for your encouragement. From what I hear so far I also really like this amp (probably better than my B1). That's why I am determined to get it right up to its full potential.

I have a toy scope (some entry level hantek). My last attempt to use it was a bit discouraging as it was not sensitive enough. Anyway, I probably should give it a try with this one...
 
Hi Wintermute,

The speakers are Troels Gravesen's Vifa C17 d'Appolito. Probably honest 95dB.
The power amp is Mini Aleph (which is 20dB if I remember the values of the negative feedback correctly (100k/ 10k||68k) which is pretty high in my setup).

As I have said in one of the earlier posts I lower the gain of Juma's preamp to below 6dB.
I can barely hear the hiss from my listening position. It is clearly heard some 20-30cm from the tweeter. I do not believe that a properly build version would exhibit this behaviour. The B1 (ok, no gain) produces almost no change in the noise level when on/off with my ear a few cm's from the tweeter.
 
... I lower the gain of Juma's preamp to below 6dB.....
That's not going to work very good. It was meant to add the gain and you don't need it with mini-A (20 dB gain) and 95 dB sensitive speakers.
What you need is a buffer that sounds better than B1. There's many of them, one that you can you easily make is this one (first sch. in post #66, buffer is on the right hand side of C7):
http://www.diyaudio.com/forums/pass-labs/244106-lsk-pre-baf-2013-a-2.html#post3682031
That buffer will work at lower voltage too (say +/-12 or 15V) but in that case the value of R19 has to be increased so that we get about 3-4V at the base of Q7.
 
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Fair enough. I kind of new it before starting this project, but I wanted to try something with bf862 and cascode. I'm pretty sure I can use it in the future.

Do you think the noise is just too much gain? Shall I chase it? I still think something is not working properly.

I will certainly look at the buffer you suggested.
 
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OK Just trying to get a feel for the environment. You have quite efficient speakers so any inherent noise will be much more obvious than with a lesser efficiency speaker.

I think getting some measurements with the scope as suggested is the best plan of action :)

edit: got distracted answering that. Looks like it's not for your setup. Might be worth having a read of Pano's article on gain structure http://www.diyaudio.com/forums/diyaudio-com-articles/186018-what-gain-structure.html

Tony.
 
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