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Old 24th Jun 2017, 8:02 am   #181
Radio Wrangler
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Default Re: Op Amp preamp for a single stage valve amp.

There are some opamp types which will work over very large supply ranges like 5 to 36v, so regulation isn't essential, though with power harvested from heaters or high rails, care is needed to protect the opamp from faults or spikes.

In using a single-rail opamp, you'll need a couple of resistors to split the supply and bias the circuit so the amplifier is running with its output near half-supply, along with DC blocks on the input and output. That half-supply resistor pair would also benefit from some smoothing to stop hum being injected.

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Old 5th Jul 2017, 7:50 pm   #182
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Default Re: Op Amp preamp for a single stage valve amp.

Hi guys,

Sorry for the late reply I've been super busy and not had a chance to do this yet.
I have a Velleman K1803 and I'm happy to take the power from the existing amp if I can.

Many thanks.
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Old 5th Jul 2017, 10:58 pm   #183
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Default Re: Op Amp preamp for a single stage valve amp.

That amp is spot on for single rail operation complete with decoupled rail splitter and all.

It needs 10-30V so robbing from the cathode isn't on, but a simple resistor zener capacitor dropper from the HT supply would be. It only needs a couple of mA plus any output load current.

For 200-250V HT use a 56K 2W resistor, a 22V 400mW zener and a 100uF 40V cap.

For crystal/ceramic cartridge input I'd up R1, 2, 5 to 1meg each and increase R7 to 22k to limit max gain to 10 instead of the 100 as standard to make setting the gain less twitchy. If 10 isn't quite enough, drop back down to 10k for 22 max gain.

Have fun and stay safe!
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Old 6th Jul 2017, 8:23 am   #184
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Default Re: Op Amp preamp for a single stage valve amp.

Thanks very much!!

I'll order the extra parts later.
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Old 6th Jul 2017, 12:39 pm   #185
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Default Re: Op Amp preamp for a single stage valve amp.

If it's a 6.3V (EL84) model then a voltage doubler off the 6.3V supply will give you what you need with no extra heat and less strain on the mains transformer.
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Old 27th Apr 2018, 1:25 pm   #186
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Default Using a K1803 op amp in a Dansette Bermuda

Hi,

Essentially I wanted to replace the cartridge and stylus in a Dansette Bermuda and put in a modern stereo one the output of which is lower than the original.

I have a Velleman K1803 op amp kit and some MMSZ3V3T1G zener diodes. I would like to take power from the existing amp if at all possible?

Anyone got any ideas about how to go about this?

Many thanks.

Last edited by Station X; 27th Apr 2018 at 1:43 pm. Reason: Threads merged.
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Old 27th Apr 2018, 2:59 pm   #187
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Default Re: Solid state preamp for EL84

You want to replace cartridge to ceramic or magnetic?

if for ceramic , just find replacement like SC12H (will be fine for EL84 one valve with only a slight downturn in volume)
if for magnetic you need RIAA preamp
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Old 27th Apr 2018, 3:24 pm   #188
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Default Re: Solid state preamp for EL84

Hello,

I'm replacing it for a ceramic cartridge that has a lower output than the original. The amp is a single stage EL84 so I want to boost the output of the cartridge by installing a Velleman k1018 op amp which needs 10 to 30v.

I want to power it by taking power from the EL84 amp. Somebody in a previous post mentioned using a zener diode, resistor and cap to do this. I need help in doing this.

Many thanks.
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Old 27th Apr 2018, 7:52 pm   #189
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Default Re: Solid state preamp for EL84

Hi Tim, if you know the current requirements of the Vellman amp, add about 5mA for the zener. You know the valve HT supply voltage and the Vellman supply voltage, calculate dropper resistor.

Add E cap and RF bypass cap (say 0.1uF) and you have your power supply.

Ed
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Old 28th Apr 2018, 3:00 pm   #190
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Default Re: Solid state preamp for EL84

Thanks Ed.
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Old 2nd May 2018, 1:29 pm   #191
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Default Re: Solid state preamp for EL84

Quote:
Originally Posted by Ed_Dinning View Post
Hi Tim, if you know the current requirements of the Vellman amp, add about 5mA for the zener. You know the valve HT supply voltage and the Vellman supply voltage, calculate dropper resistor.

Add E cap and RF bypass cap (say 0.1uF) and you have your power supply.

Ed
Hi,

I'm not sure how to wire this all up, could someone draw me a circuit?

Many thanks,
Tim.
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Old 2nd May 2018, 1:49 pm   #192
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Default Re: Solid state preamp for EL84

Hello, what Ed is suggesting looks like this...

I'm taking it that you're pulling this arrangement off the HT+ rail?

You'll need to know the (probably very small) current of your pre-amp before you can calculate the series R - series R will likely be quite high.

The electrolytic isn't shown polarised, but watch for that.

Also there's no load shown. Best to do your measurements with a load equivalent to that of your circuit.
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Old 2nd May 2018, 1:55 pm   #193
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Default Re: Solid state preamp for EL84

Quote:
Originally Posted by astral highway View Post
Hello, what Ed is suggesting looks like this...

I'm taking it that you're pulling this arrangement off the HT+ rail?

You'll need to know the (probably very small) current of your pre-amp before you can calculate the series R.

The electrolytic isn't shown polarised, but watch for that.
Thanks a lot, that's very helpful. I know the current requirements.

Thanks again,
Tim.
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Old 2nd May 2018, 2:59 pm   #194
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Default Re: Solid state preamp for EL84

Quote:
Originally Posted by astral highway View Post
Hello, what Ed is suggesting looks like this...

I'm taking it that you're pulling this arrangement off the HT+ rail?

Also there's no load shown. Best to do your measurements with a load equivalent to that of your circuit.
Yes, I am taking it from the H.T rail.

How would I work out the equivalent resistance?

Many thanks for all your help.
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Old 2nd May 2018, 3:39 pm   #195
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Default Re: Solid state preamp for EL84

Hello,

It's a long old thread but Chris up there in post no 183 has it...
''...For 200-250V HT use a 56K 2W resistor, a 22V 400mW zener and a 100uF 40V cap''
So there's all your values, ready made by Chris - they're all standard values. You'll need the rating (2W) that Chris indicated.

And then you just need the de-coupling cap shown.
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Old 2nd May 2018, 7:50 pm   #196
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Default Re: Solid state preamp for EL84

Thanks Al, a good explanation.

Ed
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Old 2nd May 2018, 9:02 pm   #197
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Default Re: Solid state preamp for EL84

Hi guys,

Thanks for all your help. I got it working nicely but I want to try and understand how to work stuff like this out in the future.

I tried to work out the correct value of dropper resistor myself but went round in circles with it.

Would you be kind enough to talk me through it?

Really appreciate all the help.
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Old 3rd May 2018, 12:11 am   #198
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Default Re: Solid state preamp for EL84

Right....

Add up the load current IL in amps plus 0.005A for the zener, call this IR (resistor current).

Subtract the zener voltage from the HT voltage to give the resistor voltage VR.

Resistor value in ohms = VR/IR

Resistor dissipation in watts = VR*IR

Zener dissipation (worst case with no preamp connected) will be VZ*IR watts.
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Old 3rd May 2018, 2:53 pm   #199
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Default Re: Solid state preamp for EL84

Hi all,

Thanks for taking the time to help me and thanks for talking me through the maths.

It sounds great and is relatively cheap to make.

Thanks again,
Tim.
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