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Old 14th Mar 2016, 8:16 pm   #61
ms660
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Default Re: Solid state preamp for EL84

Had a listen, difficult to hear LF on my PC... Hum modulation?

Lawrence.
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Old 14th Mar 2016, 8:26 pm   #62
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Default Re: Solid state preamp for EL84

I think you might be right.
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Old 15th Mar 2016, 1:04 am   #63
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Default Re: Solid state preamp for EL84

Yes, a very disappointing result for you (and us) so far. What seemed such a simple fix has now run to 4 pages already. Maybe just put your hand in your pocket and import a high-output cart from the US? Edward.
P.S. Watch out for that bolt coming out from the cartridge mounting, it might foul records when stacked.
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Old 15th Mar 2016, 8:41 am   #64
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Default Re: Solid state preamp for EL84

There has got to be a way to do it!
I'm not giving up yet.
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Old 15th Mar 2016, 9:51 am   #65
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Default Re: Solid state preamp for EL84

How about a circuit diagram of what you've built and what it's connected to?

David
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Old 15th Mar 2016, 9:58 am   #66
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Default Re: Solid state preamp for EL84

Looking at the video it looks like a recipe for hum pickup, there's not a lot in the preamp, maybe stuff it in a small metal tin/box, any hum from the HT supply should be fixable.

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Old 15th Mar 2016, 10:13 am   #67
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Default Re: Solid state preamp for EL84

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Originally Posted by Radio Wrangler View Post
How about a circuit diagram of what you've built and what it's connected to?
Hello,

it's Chris's design from post 19 and it's connected to a Dansette Bermuda (single stage EL84 amp).

I'm going to solder it all in properly today and cut back all leads as short as possible then I'll shield it all in a metal box.

I'll see if that makes any difference.

As I say I do think this design has potential but I'm leaning more toward the op amp design. I think the op amp design will in all be less hassle.

I do want to try and reduce this hum though.

Will get back later.

Tim.
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Old 15th Mar 2016, 12:10 pm   #68
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Default Re: Solid state preamp for EL84

Hi all,

I really do think it's smoothing.

I've soldered all the leads in with shielded cable. I've disconnected the Earth on the signal coming into the preamp so it's not an earth loop. I've encased it in a metal box all to no avail.

https://m.youtube.com/watch?v=zl4HWC...ature=youtu.be

I've uploaded another video.

The crocodile clip in the video is the earth for the preamp I wanted to get it as far away from the valve amp as I could. Proximity didn't make any difference.

Can we reduce the gain of this design and add some additional D.C filter?
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Old 15th Mar 2016, 12:17 pm   #69
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Default Re: Solid state preamp for EL84

Yeah, that's wide open to hum on the HT feed. The transistor acts as a current sink, controlled by the cartridge voltage, so the current times the collector resistor (150k) gives a signal voltage across the 150k, so any hum on the 250v HT is added straight into the output of the little preamp.

So stick an extra 10k in the top of the 150k collector resistor and decouple the junction with something like 10uf (330v working)

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Old 15th Mar 2016, 12:37 pm   #70
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Default Re: Solid state preamp for EL84

Quote:
Originally Posted by Radio Wrangler View Post
decouple the junction with something like 10uf (330v working)
Hi Dave,

Am sorry to be a dimwit but do you mean the base collector junction or do you mean the junction of the 10k and 150k resistors?

Many thanks.
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Old 15th Mar 2016, 12:39 pm   #71
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Default Re: Solid state preamp for EL84

The junction of the two resistors is where the decoupling cap. should be fitted.
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Old 15th Mar 2016, 12:40 pm   #72
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Thanks.
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Old 15th Mar 2016, 1:00 pm   #73
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Default Re: Solid state preamp for EL84

I don't think that circuit will give much gain because the first stages gain is used up by the RIAA EQ negative feedback and the second stage has no gain anyway.
I am going to experiment myself as I have the same problems with low output ceramic cartridges.
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Old 15th Mar 2016, 1:11 pm   #74
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Default Re: Solid state preamp for EL84

I don't know if I've done this right but I connected the positive wire of the 10 uf cap to the H.T feed connected to the new 10k resistor.
I then connected the negative to the bottom of the 150k resistor the part that's connected to the 10m resistor.

It didn't make any difference it still hummed and it stopped the amp working.

Have I done this right?
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Old 15th Mar 2016, 1:21 pm   #75
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Default Re: Solid state preamp for EL84

There have been a few circuits posted, which is why I asked Tim just which one he'd built.

It's the circuit in post #19, which is a single stage, single transistor with a bootstrapped base bias feed to raise its input Z. There is no RIAA equalisation (piezo cartridges don't need it). The problem with it is that the output signal voltage is developed across a resistor dangling down from the HT supply, so any noise on the HT gets added to the signal output to the EL84 grid.

As livewire says, a decoupler is needed at the top of the 150k collector resistor, and the HT needs feeding to it via about 10k Ohms. 10u and 10k give a roll-off at 1.59Hz so it should give about 30dB reduction in hum passing down this route.

EDIT, just seen Tim's post #74... No, the negative end of the capacitor goes to the signal ground. I think you'll need a new transistor now. The old one will have had about 250v on it while the capacitor charged up.

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Old 15th Mar 2016, 1:21 pm   #76
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Default Re: Solid state preamp for EL84

Bottom of cap to bottom rail, chassis, T1 emitter.
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Old 15th Mar 2016, 3:07 pm   #77
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Default Re: Solid state preamp for EL84

It worked!!

It's sorted out the hum!!

There seems to be a bit of bass modulation and a bit of distortion.

Is there anything that can be done to reduce bass?

I think the preamp is clipping the valve.

How can I reduce the gain of the preamp?

Last edited by vampyretim; 15th Mar 2016 at 3:24 pm.
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Old 15th Mar 2016, 3:29 pm   #78
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Default Re: Solid state preamp for EL84

The thing is, you have an RIAA circuit when you don't need one. Remove the capacitors between the base and collector of T1
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Old 15th Mar 2016, 3:47 pm   #79
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Default Re: Solid state preamp for EL84

Peter, you are referring to the wrong circuit. See the one attached to post #19.

Tim, you probably need to attenuate the input. Try fitting a 2.2M resistor in series with the cartridge. You may need to experiment with the value.
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Old 15th Mar 2016, 4:11 pm   #80
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Default Re: Solid state preamp for EL84

>It worked!!

>It's sorted out the hum!!

Good-O Thought that looked to be how it would get in.

>There seems to be a bit of bass modulation and a bit of distortion.

Not surprising. The bias arrangement is a bit crude. We don't know whether the transistor is biased with enough headroom in both directions.

>Is there anything that can be done to reduce bass?

Yes. Reducing the value of the coupling capacitor from the output of the preamp into the volume control would be a good place. Doing anything on the input to the preamp is not so good because the input of the preamp isn't well defined.

>I think the preamp is clipping the valve.

Unlikely. It's more likely that the preamp itself is clipping. You have a volume control between the preamp and the valve. If the preamp is clipping or distorting, winding the volume down will give you a quieter but still distorted sound. If it's the valve that's distorting, winding down the volume will give you a quieter sound and the distortion will diminish.

I'd suspect the bias arrangement. Done that way it depends somewhat on the transistor Hfe current gain, and Hfe can vary a lot from device to device. One check is to measure the collector voltage, but you'll need a high impedance voltmeter for this, not the 20kOhm/volt moving coil jobs. A decent DVM would be OK. Best of all would be an oscilloscope, then you could SEE what range the voltages swing over and likely even see the distortion itself.

>How can I reduce the gain of the preamp?

Find out whether you need to first. Maybe change the base bias network (costs one more resistor but in much more predictable) Then if necessary, increase the emitter resistor.

David
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