DIY Audio Projects Forum

"Lacewood" Amp V2.0
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Author:  john55 [ 11 Apr 2019, 11:06 ]
Post subject:  Re: "Lacewood" Amp V2.0

Last year I built a Lacewood V2 and mounted a line stage, tone stack preamp on top. The line stage, tone stack preamp had its own power supply so I could put it in front of other amplifiers, should I want to.
A friend would like me to build him an all in one unit. The power supply for the Lacewood V2 has a Hammond 270FX transformer and the 6.3v filament winding is rated at 5A. The total filament draw for the Lacewood V2 plus the two 12AU7s' needed for the tone stack preamp is 2.325A so I am well within limits. My question is, will the 150mA rating for the H.T. be enough for the Lacewood V2 and the tone stack preamp? I'm sure I read somewhere in this thread that the Lacewood V2 draws just under 1A.
(Note: I use a Hammond 370FX, as here in Spain I have 230V mains supply).

Author:  Suncalc [ 11 Apr 2019, 19:08 ]
Post subject:  Re: "Lacewood" Amp V2.0

john55 wrote:
My question is, will the 150mA rating for the H.T. be enough for the Lacewood V2 and the tone stack preamp?
Yes. The Lacewood V2 draws about 86mA for both channels. The preamp will only draw a handful more.

The bigger issues of which to be aware is that you need to be very careful that the preamp and power amp legs of the power supply have good filter isolation. Otherwise you could run into issues. In general this means an extra stage of power supply filtering for the preamp.

Author:  ILoveHiFi [ 12 Apr 2019, 06:58 ]
Post subject:  Re: "Lacewood" Amp V2.0

Seperate diodes or valve rectifiers can be used to fully isolate the voltage ripple from the power amp and pre amp high voltage supply.
Two diodes or 4 depending on transformer used for power amp, then directly from the same transformer ac rails you have another 2-4 diodes in parallel but the dc output is isolated and connected to its own filtering capacitor for pre amp section.

I did this is a +-12v hybrid pre amp I designed and fully isolated the buck converter from the +-12v rails.
EDIT: Buck converter for valve heater, as easiest way to get locked on 6.3v and can run heatsinkless with 1.2A output buying just cheapest ones from aliexpress

Author:  john55 [ 12 Apr 2019, 11:20 ]
Post subject:  Re: "Lacewood" Amp V2.0

Matt, can it be done as easily as this or would it be better to run a left and right version to keep things balanced?

Author:  ILoveHiFi [ 12 Apr 2019, 18:17 ]
Post subject:  Re: "Lacewood" Amp V2.0

You can't just put 20k and expect to get 265v at tone stack pre amp.
You have to find out the operating current of pre amp then do V = IR to find the actuall voltage after filtering

Author:  Suncalc [ 12 Apr 2019, 19:03 ]
Post subject:  Re: "Lacewood" Amp V2.0

It can be done as easily as that but ILoveHiFi is correct; you need to calculate the drop. The tone stack preamp with cathode follower maxes out at about 7mA for two channels. Assuming a 20kΩ dropper would give a drop of about 140v (20000Ωx0.007A) which is far too large. You would want to shoot for about 40v or so. This would indicate a resistance of about 40v/0.007A ≈ 5.7kΩ (5.6kΩ is a standard value). If you wanted to separate the preamp channels then the current drops in half and the resistance doubles.

The 5.6kΩ/47µf cap pair yields about 44dB of ripple isolation at 100Hz ripple frequency (2x50Hz). If you separated the preamp supplies left and right, the current halves and the resistor should be doubled to 11.2kΩ (11.5kΩ s a standard value) This filter stage (i.e. 11.5kΩ/47µf) would yield about 51dB of ripple isolation.

Either one of these solutions should be more than acceptable.

Just make sure you watch for all the normal pitfalls (ground loops and such). This is a complicated build. Use a BIG chassis.

Author:  john55 [ 13 Apr 2019, 05:35 ]
Post subject:  Re: "Lacewood" Amp V2.0

Thanks Ilovehifi, Matt,
Another light bulb moment for me.
Would it be okay to build as shown here Matt. I read that you prefer a 300V B+ for the tone stack preamp.

Author:  Suncalc [ 13 Apr 2019, 10:46 ]
Post subject:  Re: "Lacewood" Amp V2.0

Yes. That should work.

With the 2.7kΩ/47µf stage you get about 38dB of ripple isolation. With the other filter stage isolation to the drivers (20kΩ/47µf, ≈55dB @100Hz) and the large final capacitor at the power stages (100µf, Xc≈16Ω @100Hz) this should be sufficient to give very clean power to the preamp and to inhibit B+ feedback.

Author:  john55 [ 13 Apr 2019, 11:10 ]
Post subject:  Re: "Lacewood" Amp V2.0

Thanks for your time Matt. I don't understand how you calculate ripple isolation, nor how to inhibit B+ feedback.I think I'll leave those questions for a later date. Just happy to say I've learnt something today.

Author:  john55 [ 08 Jan 2020, 13:17 ]
Post subject:  Re: "Lacewood" Amp V2.0

My wife turned up with a 1947 Marconiphone radio the other day. She thought I might want to try to get it working.
I don't have much interest or knowledge in radio but decided to give it a look over. I managed to salvage the little mono amp that I can play with later, but as for the receiver..... I didn't have a chance (I don't know about the USA but here in Spain I don't think anyone broadcasts on SW / MW / LW anyway).
Then I looked at the case, dull brown bakelite but not a scatch or crack on it anywhere. A bit of soapy water and dasboard polish and look what I've got.
This is going to house the 6V6 and tone stack that I've been asking questions about in the last two or three posts on this thread. It might be a bit of a squash but I've got height as well as width and depth. Any thoughts?

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