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Old 9th July 2006, 08:24 AM   #1
colinB is offline colinB  United Kingdom
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Default First Post - Hum on 5687 Pre-amp

This is my first post here and Iím looking for some help with two problems with a valve pre-amplifier I recently put to-gether. The two problems are hum and too much gainÖ

Firstly, a bit of background Ė Iím a complete newbie to valve amplifiers and Iím doing my best to increase my knowledge by reading Morgan Jones book (Valve amplifiers) and looking at various websites. Over the last couple of months I decided to go ahead and build a pre-amplifier which was fairly quickly followed by a power amplifier.

So, the pre-amp was based on a 5687 valve as per the schematic on www.diy paradise.com which appeared to be relatively Ďsimpleí to build for a newbie. It took me some time to complete but Iím new to this so no big deal! Well after testing to find all voltages were approx. correct everything looked great. However, at this stage I had no power amp so I couldnít really test the preamp playing music.

So I decided to build a power amp. This time with some help with a schematic I built a EL34 SE power amp (using ECC83 drivers). Again, everything went ok Ė no problems with voltages on power amp, etc. Excitedly I coupled the pre-amp and power amp together but it was immediately obvious I had problems Ė loud hum.
Played some music anyway but then second problem surfaced Ė too much gain (volume was too loud before 9í o clock on dial.)

Some investigation was carried out: Could play music through power amp (with Walkman CD + volume control) and there was no hum so I figured that this meant the pre-amp was at fault.

So looking at pre-amp
Using Hammond power TX (373X) and 5Y3 rectifier then 30H choke/50uf cap/30H choke/50uf Cap to give B+ of ~250V. Heaters on 5687 valve use AC so cables are twisted together.
Tried to keep signal cable away from AC and B+ as much as possible and used screened cable where possible on signal side.
Phono connectors are isolated from chassis so earthing (which proved problematic for me) is connected to chassis at one of bolts under mains transformer. This could well be wrong as I am not too sure what is best.

I only have a DMM so canít carry out much testing to explain type of hum however it is there when volume is at minimum and is largely volume independent (although once volume pot is at ~1-2oíclock without music there is an increasing buzzing noise on both channels as well as the hum).

Iíve tried re-routing earth connections and have tried different combinations of connecting screen of signal cable to earth (e.g. at one end only, at both ends etc) but hum still persists.

Only way forward maybe to go DC on heaters (I think Bibster did this when he had similar problems with hum on a 5687 amp) but Iím not sure if my transformer (6.3VAC @ 4A) is up to spec for this change and Iím pretty sure Iíd need a schematic to show me how this would be best achieved. Hopefully the hum is coming from the layout rather than heaters as the solution should be easier.

Iíve attached a photo of schematic from which it will be hopefully it will be obvious where the faults lie (to an expert eye Ė unlike mine).

For the gain problem I tried to remove the cathode cap but this really didnít have any noticeable effect. Also put 150k resistor on each channel of input signal prior to Alpha (100k) volume pot which seems to have helped marginally Ė can now run at 9ío clock on pot without fear of damaging Ďspeakers.)
Maybe I need to change the pot? Does anyone know if this is normally an issue with this 5697 pre-amp?

Any help would be gratefully appreciated - this hum problem is really driving me mad?
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Old 9th July 2006, 08:43 AM   #2
arnoldc is offline arnoldc  Philippines
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Check out bibster's 5687 linestage topic to inspire you
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Old 9th July 2006, 08:51 AM   #3
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Regarding the hum all the information you will need is in the Bibster thread.
You can just about get enough DC out of a 6.3V winding if you use a big initial filter cap and basically just rectify it and wack the big cap on the supply. It will run the filament a little starved, but this is a good thing in my book. In my opinion a quick and dirt lash up of this might save a hell of a lot of waisted time.
Have you earthed one side of your filament winding ? Did you lift the filament to +20V as per the schematic ?

Shoog
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Old 9th July 2006, 09:10 AM   #4
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Regarding your gain problem. The diyparadise linestage doesn't have excessive gain. Therefore your gain problem lies firmly with your choice of driver in your power amp.
You have two solutions;
1-change the driver stage (the best solution)
2-put a voltage divider on the input of the amp. Use Metal Film to minimise the the sonic impact. 100K from the phono plug followed by another 100K to ground should reduce your gain.

The excessive gain of your amplifier is amplifying the hum of your line stage.

Shoog
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Old 9th July 2006, 09:36 AM   #5
colinB is offline colinB  United Kingdom
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Hi Shoog,

Heater supply is earthed via CT from Transformer - so that is ok. Filament isn't lifted +20V though at the moment. Must re-read and make notes from Bibsters thread to find out how to do this.
Regarding going to DC should be able to put something together later (assuming parts are available etc).

Thanks
Colin
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Old 9th July 2006, 11:40 AM   #6
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Quote:
Regarding your gain problem. The diyparadise linestage doesn't have excessive gain. Therefore your gain problem lies firmly with your choice of driver in your power amp.
Well, I'm not going to say that I buy this statement without further examination. Do you have another power amplifier in which to try your line stage with? If you don't have one can you borrow one from a neighbor or friend?

You mentioned that you have a multimeter is this correct? If so measure the output of your linestage on both the DC and AC millivolt scale and post the results.

I am suggesting an examination of both parties (linestage and SE amplifier)before you dive in and start tearing the hell out of something that may or may not be the problem. Make a few tests and come back and post your results. If you have a scope look at the output of the line stage.
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Old 9th July 2006, 12:24 PM   #7
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It maybe that the power amp is designed for a CD level input and as such any gain will be excessive. If thats the case then the linestage could be considered wrong for the application. The diyparadise preamp is a moderate gain preamp.
Whatever the case the marriage needs some slight modification at the least, padding down the input is the most straightforward approach.

Sort out the hum first.

Shoog
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Old 9th July 2006, 12:47 PM   #8
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A volume control on the input of the SE amplifier might be in order regardless.

Are you using a 3 wire grounded power cord? If not I suggest modifying your line stage and amplifiers. The cost of some cords is less than a funeral.
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Old 9th July 2006, 09:35 PM   #9
colinB is offline colinB  United Kingdom
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Finally back after fiddling with bridge and caps all afternoon in an attempt to overcome the hum problems.

Thanks for suggestions from Shoog & burned fingers - maybe I can answer some of your points.

Regarding the hum eventually tried following combination

6.3VAC/Bridge/4700uF/0.56R(10W)/4700uF

this gave DC of ~6.15V measured at valve socket - ok!

Having only DMM available (no 'scope) unable to measure any changes in the hum with AC v's DC filament heating but my ears tell me the hum is greatly reduced.

The only problem now might be fitting the extra components into chassis.

Regarding the high gain I haven't made any changes to the power amp as suggested yet (by Shoog i.e voltage divider on input but will try this later/to-morrow). This seems better than changing the driver in the power amp at this point.

I'm a little bit worried by the marriage of the line stage amp and power amp. Whilst testing I use a Walkman CD which has a line out which I connect to line stage or I can use headphone output/vol control connect directly to the power amp. However, ultimately I want to use my main CD player which has no volume control hence the need for a line-stage. (I guess I could put a volume pot on the power amp as an alternative for CD playback making the linestage redundant but I certainly did have that in mind when I started out.)

Hopefully using voltage divider on the input of the power amp will solve the gain problems.

BTW I'm using 3 wire grounded power cord on both amps.

Colin.
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Old 10th July 2006, 09:28 AM   #10
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Glad to hear your've made real progress with the hum. I would be inclined to make your second capacitor at least 20,000uf to get the best DC. Because you are restricted to purely capacitive filtering, and the current draw is relatively high - you need as much as possible. You only need 10V ratings so that should still be relatively small.

A volume pot is basically a variable voltage divider but with more flexability. Still I would say that the voltage divider with MF should sound better so if you can get it working to your satisfaction thats the way to go. Inevitably you will experience a slight loss of detail. The secondary advantage will be that because the hum is preamp induced, you should kill most of the remaining hum.

Keep us posted.

Shoog
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