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Old 29th September 2003, 08:57 PM   #11
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Hi,

Quote:
Interestingly enough the hum gets real loud if I touch the chassis of either the cd player or preamp (being used with headphones)
Groundloop, I reckon.

Quote:
Out of interest how close should R7 be to the socket pins, at present I habe a long length (20-30 cms) of cable running into pcb mounted terminal block which is then connected to R7.?????
That's the cathode bias resistor which is shared between the two output valves.
Its' top should go close to the cathodes' midpoint.
IMHO, using two separate 300R cathode resistors would make life easier on you.

Maybe UA would like to comment on his design choices?

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Old 30th September 2003, 05:53 AM   #12
Raj1 is offline Raj1  United Kingdom
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Default groundloop

Hi,

Does the groundloop issue come from the output (interconnects to starearth at preamp), or more from my implementation of earthing on the pcb to the 3a5's???

At present I have a common earth wire running the length of the pcb, all negative ends from the caps etc come to this line, from here I have connected the ouput ends of this to a pcb mounted 3 terminal block. The cathode bias resitor has it's own terminal with a wire running from the block to the pins respectivley. One of the other blocks is the B+ to the output transformers. The third block supplies earth to the 4 1800pf caps. Perhaps I shold have just one earth block output to the tube pins instead of 2? I was thinking of connecting the c bias resistor close to the pins, and using the bottom end of the resistor for the earth supply to the caps also, hence negating the need for 2 wires carrying earth to the tube sockets..........

Any thoughts and advice???

Thanks
Raja
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Old 30th September 2003, 09:01 AM   #13
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Default Ground loop

Taking a look at the pdf of the circuit I noticed I did not explicitly state that one side of the tranny secondary needs to be grounded.

Click the image to open in full size.

Sorry for the blurry picture, but I only found out recently how to transfer spectra digitally to my computer.

It may save you some hum.

Sowter also recommends terminating the secondary with a resistor to dampen eventual ringing. You can try and measure or listen to a variation of terminating resistors.

I'd check the circuit for ground loops first.

Another cause for humming may be excess PSU hum combined with bad matching of the 3A5. The hum should cancel out. First measure the PSU residual hum (with a scope, Raj!), then check the matching.

About the hum, a picture says more than a thousand words. Ground loops definitely look different than PSU AC remains. Hope by now you see why you need a scope? It's hard to be of any assitance since I don't know what I'm looking at.

Weak bass may be a result of poor DC matching saturating the Xformer core, but then again it's also a characteristic of the Legato dac. Need some objective numbers (pictures) to decide what's what.

A cap over the cathode resistor may eliminate some mismatch, but it'll have to be quite large (> 2200 uF). It will solve difficulties arising from eventual layout problems (I once picked up radio with a cathode resistor) but that only comes into play when the layout is extremely poor (in the forementioned case, the heater wires were 5 meters long). I prefer no capacitor there (well, actually, I prefer a current sink there).

Quote:
IMHO, using two separate 300R cathode resistors would make life easier on you.
Would be, but the cathode is internally shared between the triodes. Which indicates the midpoint should be fairly accurate, since differential operation was intended.

Remco
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Old 3rd October 2003, 05:12 PM   #14
Raj1 is offline Raj1  United Kingdom
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Default star earth

Hi,

Should I be running all of the 0v lines to star earth on the chassis (0v from the main B+ supply, also from 3a5 heater supply)?

Thanks
Raja
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Old 3rd October 2003, 05:22 PM   #15
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No!

Heater (or more correctly: filament) supply should be floating!!! This is a directly heated tube.

All else nicely grounded, shouldn't pose any problem. This is not millivolt stuff so hum is out of the question unless a ground is missing.
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Old 3rd October 2003, 05:36 PM   #16
Raj1 is offline Raj1  United Kingdom
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Hi Remco,

Okay what I'll do is attach 0v from b+ and output secondries to a single star ground which is already in the cdp.......


Remco I've seen a few scopes around that use a pc to work are these okay for this kind of work? I don't know much at all about scopes, and I've seen prices starting from say 150 all they way upto 1000. Do pc versions generally have what I would need to make measurements?


Thanks
Raja
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Old 3rd October 2003, 05:45 PM   #17
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I see you're in the UK.

Check eBay !

All you need is a 2 channel machine with reasonable sensitivity and bandwidth. Say 50 MHz and 5 mV. Go for a Tek 465 or 455 for instance.
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Old 4th October 2003, 05:50 PM   #18
Raj1 is offline Raj1  United Kingdom
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Thanks Remco for the link,

I've done the star earthing and the hum is virtually gone..........

All I'm left with is a slight hf hiss thru one channel, which goes if I press on the chassis near the 3a5 representing that channel. I've tried it with a plastic handle also just to check I wasn't acting as a ground, the chassis is slightly warped on this side......

It's not noticable when the music's on so I may leave it.........

But I must say I'm extremley impressed with the smooth sound of this output stage, it's made my harsher sounding discs sound so..............analogue. The bass is good, I initially thought it to be weak but I figure this was due to no star earth for the ht supply, A very different listening experience to the solid stage output, which was perhaps cleaner but also more fatiguing

Well Remco do you have any plans for a tube headphone amp, perhaps SE?

Thanks
Raja
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Old 4th October 2003, 07:57 PM   #19
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The hiss sounds like a bad socket or dirty tube pins. Try cleaning those and check the sockets for firmness. The easiest check is to swap the 3A5's and see if the problem moves to the other channel. Could also be a bad solder joint btw.

Can't say more on the hum without some measurements, I'm afraid. You could try increasing psu buffer capacity, but be aware of the AZ1 internal resistance. Try psud2 simulations.

Glad you like the sound. I recognize and acknowledge the comments you made.

A headphone amp is not in the planning. But if your headphones are 300 Ohms or higher, try driving them directly from the CD enhancer II output. Rout is about 100 Ohms. I could drive even my 32 Ohms Sony headphones to quite reasonable, undistorted power levels. You'd only miss a volume control but could add one in between the dacs and the tube stage.
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