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Old 7th April 2013, 02:20 PM   #1511
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Default Heater noise

Quote:
Originally Posted by apelizzo View Post
With regards to the heater I think I know what you are going to tell me... Yes the heater is in series because I could grab the 12VAC from the 12+12 transformer (power stage transformer), reducing the current and the section of the interconnecting wire between the cases. I eliminated the noise by adding a 475nF cap between one end of the heater and signal ground. Between signal ground and earth I connected 12Ohm resistor in parallel with 100nF capacitor.
The noise is not completely gone though. I think is the problem with the heaters in series ?
For my tests I used 12Vac and connected the heater centre tap to ground.

I suspect in fact that the residual hum is not so easily removed, as it can come from several different sources.

Is it 50Hz or 100Hz?

If it's 100Hz then it can be from the B+ supply since it would be from full wave rectification.

If it's 50Hz then it's more likely the heaters.

Of course, one needs to check the input that there isn't hum or an earth loop from the source. I assume the issue is there with the input shorted.

I am guessing that you have already covered all these bases.

Also what biasing do you have for the 6C33C tube?

I did have to check a few out to get a couple similar, and I had one that was not good at all (probably gone a bit soft).

Best wishes,
Susan.
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Old 8th April 2013, 12:18 PM   #1512
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Default Residual Noise

When you tested the line stage did you have the same issue with the heater? Did you resolve the issue with grounding the centre tap?
The 6c33c is self biased with the transformer own DC resistance. The current was 285mA with brand new tube. I just let the system break in for a while.
The tube's B+ is 37V and the residual ripple is about 2mV p-p that becomes 20mV at the gates. That is probably enough to hear it?

The negative of the two DC rails are joint inside the amplifier case and not in the power supply case.

The line stage transformer output and the input transformer are connected in single end. If I connect them in balanced the noise figure goes up. In balance mode the centre tap of the line stage transformer is grounded. Now both transformer are grounded at one end. That gives me a little more extension in HF because it reduces interwinding capacitance: inside the transformer the ground points are at the same starting point of the primary and secondary.

Last edited by apelizzo; 8th April 2013 at 12:25 PM.
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Old 8th April 2013, 12:30 PM   #1513
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Default Residual Noise

I will start troubleshooting the noise tomorrow. I will temporary supply 12V DC from an external DC power supply to eliminate the heater chance to generate noise. I will then deal with the ripple on B+. Probably I need to make a two stage electronic choke.
As I said the power stage is totally quiet as the ripple is cancelled in push pull configuration. But also in order to cutoff harmonics the power supply is a CLC. The choke smoothes down to a nice round ripple supplied to the 2 drains.
The B+ for the tube is also a CLC but the L is electronic with one mosfet and I think a single stage is not good enough.

Last edited by apelizzo; 8th April 2013 at 12:38 PM.
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Old 8th April 2013, 12:55 PM   #1514
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Quote:
Originally Posted by kukayron03 View Post
ma'am / sir ...,

do you think stk4050 can drive a 1pc of 18" sub??????

the power of stk 4050 is 200W RMS in the cathalog i dont think so if its real?

i wanted to build a subwoofer amp. that can drive 18" inch sub that its have low cost of building and its have a good quality of sound plz give me any suggestion .

tnx.
Consider also Hypex Class D modules if you want to mount the amp inside the sub. UCD400 or 700

Last edited by apelizzo; 8th April 2013 at 12:59 PM.
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Old 8th April 2013, 01:24 PM   #1515
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Default Interstage transformer

That's my winding machine built from scrap wood. The counter is a $2 calculator connected to a switch on the wheel.
The wire is connected to the = button. So just press 1+ and then the = pulse will increment by one. If you mess up you can do 1- and counts backwards. Hahaha

Second attempt of making the transformer. I made the bobbin in paper which is better then plastic. Unfortunately after 2 weeks of work the winding arrangement did not give best performance.
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Last edited by apelizzo; 8th April 2013 at 01:46 PM.
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Old 9th April 2013, 08:15 AM   #1516
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Quote:
Originally Posted by apelizzo View Post
That's my winding machine built from scrap wood. The counter is a $2 calculator connected to a switch on the wheel.
The wire is connected to the = button. So just press 1+ and then the = pulse will increment by one. If you mess up you can do 1- and counts backwards. Hahaha

Second attempt of making the transformer. I made the bobbin in paper which is better then plastic. Unfortunately after 2 weeks of work the winding arrangement did not give best performance.
The winding looks quite good, congratulations. What material you are using for winding insulation, what cores ...?

As filament supply i'd recommend to try out unregulated dc as well (use small 3mH inductors in positive and negative rail).
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Old 9th April 2013, 07:09 PM   #1517
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Hi,

Back at work after Easter holiday break...

Quote:
Originally Posted by apelizzo View Post
When you tested the line stage did you have the same issue with the heater? Did you resolve the issue with grounding the centre tap?
The 6c33c is self biased with the transformer own DC resistance. The current was 285mA with brand new tube. I just let the system break in for a while.
The tube's B+ is 37V and the residual ripple is about 2mV p-p that becomes 20mV at the gates. That is probably enough to hear it?

The negative of the two DC rails are joint inside the amplifier case and not in the power supply case.

The line stage transformer output and the input transformer are connected in single end. If I connect them in balanced the noise figure goes up. In balance mode the centre tap of the line stage transformer is grounded. Now both transformer are grounded at one end. That gives me a little more extension in HF because it reduces interwinding capacitance: inside the transformer the ground points are at the same starting point of the primary and secondary.
When I was testing I used an AC heater supply, with the centre of the tube to ground.

However I was using my bench PSU for the HV B+ (although at 37 volts the term HV is perhaps a bit of a misnomer in this case).

My testing found that the 6C33C tubes were not happy above about 300mA, getting much noisier (not sure what the B+ was at that point though).

Perhaps you can use the 37 volts with a low drop out regulator down to say 34 volts and see if that sort the issue.

I would be thinking of running the 6C33C at around 250 mA. However my testing was a while ago so I really need to have another look at this.

Also I have the secondary of the SE line driver transformer switched with a reed relay that is opened when volume is muted to removed any turn on/off thumps. Not sure if you need this in your set-up as the power supplies are all together.

Best wishes,
Susan.
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Old 11th April 2013, 02:39 PM   #1518
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Quote:
Originally Posted by Joao@AltheaMusica View Post
The winding looks quite good, congratulations. What material you are using for winding insulation, what cores ...?

As filament supply i'd recommend to try out unregulated dc as well (use small 3mH inductors in positive and negative rail).
Thanks. The material of the former was insulating paper for transformer and the side wings and central divider hmmm a nice natural paper from a cereal box.

I can try with DC but I try to avoid it as I prefer the valve sound with AC heater. With DC heater IMO the the cathode generates a space charge that is polarised and at the end the valve sounds dead. This is just my opinion.
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Old 11th April 2013, 02:45 PM   #1519
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I am getting addicted to the sound of this amplifier. The sound is incredibly detailed, fast and yet liquid. Transformers are magic things....

BTW I am listening with my speakers with 15" FTR Celestion, Beyma CP380M compression driver and waveguide with 107dB efficiency and with my ear dangerously 10cm close to the throat of the waveguide I cannot hear any hiss at all. So this amp is really quiet.

Last edited by apelizzo; 11th April 2013 at 03:00 PM.
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Old 12th April 2013, 08:48 AM   #1520
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Quote:
Originally Posted by apelizzo View Post
Thanks. The material of the former was insulating paper for transformer and the side wings and central divider hmmm a nice natural paper from a cereal box.

I can try with DC but I try to avoid it as I prefer the valve sound with AC heater. With DC heater IMO the the cathode generates a space charge that is polarised and at the end the valve sounds dead. This is just my opinion.
If you have no noice, hassle with ac-filaments then it's ok and why then using dc... What core material you are using?

@Susan
We discussed in a fewer post of this thread a phono stage using interstage trannies. I have designed one in single ended with IS - trannies in the drain - and LCR-RIAA. Sounds impressive.
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