Cornet 2 Build Finished but some problems with voltages and noise. Advice please

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peter_g

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Hello all

I have just completed my build which overall went very well.

I have a stock build with Auricap upgrade and anodized Lansing chassis.

Some of the test voltages are a bit out of spec, the heater voltage low and I have a bit of noise as follows:
.

Nominal Supply voltage 240v – measured 236v  (UK based)



Test value v   Measured v

365      355

Left      Right

155      182      172
150      163      154
150      139      138
330      324      325
300      296      297

R223 upside   7.22v
R223 downside (heater voltage)   5.5v

I don’t have access to a scope or inverse RIAA filter at the moment so have not been able to test the output.

I have however tried it through my system and it sounds very good –even at this early stage musically better then the Trichord I have been using to date.

However there is some noise – a combination of hum and hiss (which I think is what some refer to as valve rush).

I am using an Origin Live Aladdin moving iron cartridge (a Soundsmith derivative) which has a low output (2.2mV) and therefore I need to crank it up a bit more than a normal MM with something like 5mV. This makes the system a bit more sensitive to noise and I an finding the current levels a bit distracting at medium to high listening levels during quiet passages.

So with regard the test voltages I looked at some previous posts and have tried re-flowing the solder joints on R206 but this made no difference so I will replace these to see if that helps

The low heater voltage has also been reported and I intend to change R223 to around 1.1ohm to bring the voltage up to 6.2v.

I would like to know if this is needed just to improve valve life or can I expect other improvements?

With regard to noise:

1)   I swapped the 12AX7s around and there was a noticeable improvement in both hum and hiss.  I tried this several times with the same result. So I guess I have at least one dodgy tube. These tubes are Russian made Mullards from AES. I would like to replace these with better quality tubes with a low noise floor and would be really grateful if anyone could recommend some which they have found to perform well and most importantly quietly.

I am using a JJ electronic 12AU7 and a  Sovtek 5Y36T. Again any advice on improvements would be much appreciated. This is my first valve project and I am beginning to understand  what tube rolling is all about☺



2)   I have used pcb mounted RCAs as I thought this would reduce the level of hum but if anyone has found chassis mounted jacks better I will try this. I have made sure there is no grounding between the cable jacks and the chassis and also got a good ground from my turntable. When I disconnect the input cable from the Cornet the hum reduces but there is still a hiss. I have tried the cable connected to the Cornet but not to the TT and this increases hum and is sensitive to position and reduces when I touch the jacks  but I guess this is not too surprising.



3)   I have made sure the TX is grounded well to the chassis by getting metal-to-metal contact but this did not improve things. I haven’t yet been through all the chassis panels/scews to make sure they are grounded but will do but I don't think this is a problem

So there it is – a lovely sounding amp but with just a bit too much noise in my system so I would really appreciate any ideas on how to quieten things down

If you’ve got this far through my ramblings thanks for sticking with it.

Cheers Peter

hagtech

The voltage look fine except for the heater.  You are doing the proper fix (lower resistor) to bring it back up.  I attribute this to a change in the design of the power transformer.  It seems to have gone through two revisions since I first designed this circuit.

The hiss will reduce when you connect the cartridge.  Hum is caused by many different things, but it sound like you are peeling through the onion layers nicely. 

jh

machine

The hum for me was gone when I connected the grey wire from the transformer to the common ground in the case.

This is described in various posts about hum and the Cornet2...


peter_g

  • Jr. Member
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Hi

Thanks for the feedback to you both. I will investigate the famous grey wire fix over the next few days and let you know how I get on

Peter

poty

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Some of the test voltages are a bit out of spec, the heater voltage low
You should begin with the heater voltage.
The low heater voltage has also been reported and I intend to change R223 to around 1.1ohm to bring the voltage up to 6.2v.
It can solve at least some of your problems.
However there is some noise – a combination of hum and hiss (which I think is what some refer to as valve rush).

I am using an Origin Live Aladdin moving iron cartridge (a Soundsmith derivative) which has a low output (2.2mV) and therefore I need to crank it up a bit more than a normal MM with something like 5mV. This makes the system a bit more sensitive to noise and I an finding the current levels a bit distracting at medium to high listening levels during quiet passages.
I'd try to change the loading resistor a bit, but I agree with JH - your cartridge should shunt the input and lower the noise and hum. By the way - have you tried to short out the inputs of the Cornet 2?
2)   I have used pcb mounted RCAs as I thought this would reduce the level of hum but if anyone has found chassis mounted jacks better I will try this.
I don't think this is necessary at this stage.
When I disconnect the input cable from the Cornet the hum reduces but there is still a hiss. I have tried the cable connected to the Cornet but not to the TT and this increases hum and is sensitive to position and reduces when I touch the jacks  but I guess this is not too surprising.
It may show that the cable need checking too.

peter_g

  • Jr. Member
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hello Poty

Thanks for the reply

I have done the TX grey wire mod and made sure I have got good metal contact on all chassis joints. This has made a big improvement to the hum.  However there is still a bit there which I would like to get rid of and the tube rush is still there too

How would you expect increasing the heater v to change things - a bit more gain ,less noise?

ref the loading resistance. Would increasing it increase the gain on the mid to higher frequencies. Any idea how much I should raise it to?

i have tried two good quality cables and they both seem to pick up a bit of hum when not connected to the TT. With the input cable disconnected from the Cornet there is hardly any hum but still tube noise (This is all at quite high amp gain). This doesn't change much when I connect to the cartridge.

Anyway have got the new heater resistors on order and will let you know what that does in a day or two

Many thanks

Peter

poty

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How would you expect increasing the heater v to change things - a bit more gain ,less noise?
Too small heaters voltage changes all tube's parameters. It lowers the cathode emission ability which could lead to increasing noise. The other bad outcome - the voltages and currents are out of spec.
ref the loading resistance. Would increasing it increase the gain on the mid to higher frequencies. Any idea how much I should raise it to?
I was thinking about decreasing the value, not increasing. :) Usually low-output cartridges need lower loading resistor, which helps in mitigating hum pickup from the turntable side.
i have tried two good quality cables and they both seem to pick up a bit of hum when not connected to the TT. With the input cable disconnected from the Cornet there is hardly any hum but still tube noise (This is all at quite high amp gain). This doesn't change much when I connect to the cartridge.
Both tests' results are strange for me... My interconnect does not increase the hum in the comparable phono setup. There must be noticeable decrease in both hum and noise after connecting the cartridge. Please disconnect input wires, short out the input RCAs and listen to changes.

peter_g

  • Jr. Member
  • Posts: 14
Hi

Been away for a while but now back on the case.

Heater voltage now spot on at 6.2v, and things have improved slightly. I tried shorting the inputs and find that I get more hum and more hiss than when there is no input cable connected but this is at quite high volumes. When I connect up to the TT and cartridge the hiss remains low but the hum increases. This is now almost OK but I would like to get rid of this last hum. I have also tried the Cornet in a completely different system with similar results.

I did not twist the transformer wires: maybe this would help but which wires should be twisted together?

With regard to the input load resistance any idea what sort of values should be used?

I also noticed that holding my hand close to V200 increases hum and more so when I touch it.

So once again many thanks for help so far and I would be grateful for any further ideas

Cheers

Peter

Speedskater

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  • Kevin
In the AC power stage the Send and Return (High & Low) of any current pair (or triplet) should be twisted together.
So:
a] Primary AC the Hot & Neutral (but not the Safety Ground/Protective Earth).
b] The power transformer input primary.
c] Each two terminal secondary (separately)
d] With a center tapped secondary do all three wires.

As a general rule:
All unshielded mufti-conductor cables and cords should be twisted.  This includes AC power, DC power, signal and control. 

peter_g

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Thanks
I'll give it a try

poty

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I tried shorting the inputs and find that I get more hum and more hiss than when there is no input cable connected but this is at quite high volumes.
It should be done with a short wire, which should not be touched.
When I connect up to the TT and cartridge the hiss remains low but the hum increases. This is now almost OK but I would like to get rid of this last hum. I have also tried the Cornet in a completely different system with similar results.
The increased hum is out of scope of the Cornet - check either turntables or interconnects for that. Is it possible to check one channel at a time, just to be shure the internal wirings are OK? (I mean connect only one interconnect's RCA to one of the inputs of the Cornet, listen to, disconnect, connect another channel, listen to...)
With regard to the input load resistance any idea what sort of values should be used?
I think the value to start with can be taken from the cartridge's datasheet. As far as I know Cornet has two pairs of spare holes just for that, where you can connect a resistor to try (without unsoldering the initial resistor). This tuning if only for sound, it is unlikely the loading resistor will help you with fighting the hum and hiss.
I also noticed that holding my hand close to V200 increases hum and more so when I touch it.
Well, actually this is OK. That is why the essential part of eliminating hum is shielding: you should be sure all metal parts of the case are connected to the safety ground.

peter_g

  • Jr. Member
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Hi Potty and thanks again for the reply

I have re-tried shorting the inputs as suggested : short wire  and no touching. Still the same result with similar level of hum with the TT connected.

I tried my Trichord phono amp with exactly the same set up and it is virtually silent at full volume and I also tried one channel at a time with the Cornet and no difference so I think the Cornet must be causing the hum. I will try twisting the TX wires as suggested by speed skater and re-check chassis earthing.

Has anyone changed from  pcb mounted jacks to better quality chassis mounted ones and found an improvement?- this might be another direction to follow.

Apart from this the Cornet sounds better and better as it burns in. It is difficult to explain exactly how but my over-riding impression is that is  just easier to listen to then a solid state devise - marvellous

poty

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OK!
Could you report the results of the following tests?
1. Disconnect TT. Switch on the whole rest system. Listen to noise. Switch off the Cornet (preferably do the following: turn off the volume, turn off the Cornet, turn on the volume). You should hear the noise fade away. If you still hear the hum - the problem is with some ground loops somewhere in your system.
2. After p. 1 (and still switched off) - pull out the mains plug. (Do you hear some changes?).
3. Pull out the first tube (V200), plug the mains in and switch on the Cornet. Listen to the changing in the hum.

peter_g

  • Jr. Member
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Hi Poty

Results:

1.  The noise fades away
2. No real changes the noise fades away in the same way
3. Hum and hiss reduce significantly


Thanks

poty

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1.  The noise fades away
Then the source is not further in the chain of amplification.
2. No real changes the noise fades away in the same way
This is just double check if the point 1 shows a problem.
3. Hum and hiss reduce significantly
The first stage is most sensitive of the device, so it is obvious that this part should influence on the noise more than other. This test proves that most of hum is induced. Relating to hiss... I guess you should change the first tube for something less noisy.

peter_g

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Ok - so does this suggest the source which is inducing the hum is external to the cornet or could still be from inside? an earth problem or pick up from a power lead?

Thanks

poty

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In my opinion for the hum the problem lies outside the Cornet, but I may be wrong. In this case the position of the Cornet in relation to the nearby devices should change the hum amount. It is no so about the hiss. It is inside sourced 100%.
It is no ground problems definitely. Why I think so? Because when you unplug the mains you break the ground loop through the safety ground and you have already tested the breaking with no results. The only tiny possibility - if the TT grounding wire is connected somehow to the safety ground in mains and you had not disconnected it during the experiments.
If it is inside it can be from bad shielding or the AC wires (that is why Jim asked for a photo). It can be also due to bad filtering (undervalued capacitors, some leakage somewhere, bad solder joint...).
P.S. I admin that my English is not very good, so if you can't understand something I'll try to rephrase.

peter_g

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Hi and once again thanks for taking the time to reply.

I have run the cornet in  different rooms and buildings and in a different system with similar results.  Also tried a different phono amp in my system which was virtually silent so I am thinking the problem lies within the cornet.

I had the TT ground connected to the cornet when I did the tests so I will repeat with it disconnected

I am going to work on the internals and check out everything again.  Got a few other urgent projects to do around the house (nowhere near as interesting!!) so will get back to it as soon as possible

Cheers

Peter

poty

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I have run the cornet in  different rooms and buildings and in a different system with similar results.  Also tried a different phono amp in my system which was virtually silent so I am thinking the problem lies within the cornet.
Yes and no... The ground schemes in the Cornet and another phono amp may be different. Sometimes it happens that the plain ground schemes like incorporated in the Cornet, while not bad at all themselves, can reveal problems with other devices in the chain. I know it sounds not very understandable at first glance, but there is no "universally good" grounding scheme.