2P29L Preamp

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Ray P
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#1 2P29L Preamp

Post by Ray P »

I mentioned in my MoFo thread that I need to buid a preamp with around 20dB of gain before I can go much further with it so I planned to build the 2P29L preamp as per the Bartola site:

https://www.bartola.co.uk/valves/2017/0 ... ht-preamp/

I've got most of the components but have been waiting for a transformer from Terry at Canterbury Windings (who has now partially retired BTW). The transformer arrived this week so I have started putting it together. Today I built, installed and tested the CRC power supply that is 'upstream' of the Bartola Gyrator boards along with two of Andrew's DHT filament supplies. I'll post some pictures when I have a chance.

I won't be able to complete it quickly though as I'm still waiting on some Muses volume control modules.

Incidentally, although I've labelled this as a 2P29L preamp I've tried to be a bit smarter as it also includes a BUF-03-based buffered passive preamp (using a separate Muses volume module to the 2P29L section but both controlled through the same encoder board) - the preamp will have three outputs, two from the 2P29Ls (with and without the output cap), with about 18dB of gain, and the buffered passive - my thinking is for it to be a master volume control in an open baffle context with the 2P29L stage driving the MoFo for the fullrange speaker and the buffered passive driving the chip amplifier for the bass units (the chip amp has the required gain).
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Wolfgang
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#2 Re: 2P29L Preamp

Post by Wolfgang »

Interesting approach. Basically some kind of a simple active x-over without the options for adjusting and fine tuning.
The Muse volume control adds so many opamps that one could use a good active analog x-over and would get the same impact of solid state (if that's a concern) but with all the options needed for perfectly adjusting the drivers to each other and the OB speaker to the room.

If the amps used for driving the speakers are not the same (input sensitivity/gain) it can sound totally different using active individual volume controls for each driver vs passive adjustment.
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Ray P
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#3 Re: 2P29L Preamp

Post by Ray P »

Wolfgang wrote: Tue Apr 13, 2021 4:28 pm The Muse volume control adds so many opamps...
I don't see any with my Muses boards beyond what is integrated into the Muses chip?

https://www.njr.com/electronic_device/P ... 2320_E.pdf

I have just modified my buffered passive preamp to use a Muses volume control instead of the series switched attenuator and it sounds at least on a par in front of the 13E1 SE-OTL.

Beyond that, of course, if one simply plugged it all together it would likely be a very strange-sounding result but I've put some thought into this and decided it's worth experimentation - the worst that can happen is I don't get a satisfactory balance to the sound and resort to using a crossover arrangement. My objective is to keep the fullrange driver element as umolested as possible, although it will be possible to make some adjustments, such as the crossover frequency (circa 150-180Hz) with component changes. The bass element will have at least an adjustable crossover and level control and possibly the option of some equalisation.

The first step is to finish the preamp hook it up to the bandwidth limited MoFo and measure the full-range driver frequency response whilst its mounted on a representative baffle. If that looks promising I will built a bass section and aim to explore integrating it with the full-ranger with something like MiniDSP so I can understand the possibilities.
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Nick
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#4 Re: 2P29L Preamp

Post by Nick »

Don't know what they have implemented but page 10 of the app note has a couple of op amps used as variable gain devices.
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#5 Re: 2P29L Preamp

Post by Ray P »

Nick wrote: Tue Apr 13, 2021 9:21 pm Don't know what they have implemented but page 10 of the app note has a couple of op amps used as variable gain devices.
Not present.

Excluding power supply regulators, the module has two 'active' devices, the Muses chip and it's associated microprocessor.

In the passive preamp I have already built and use the Muses volume module outputs to a BUF-03 buffer, but that's a different project to this one.
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#6 Re: 2P29L Preamp

Post by Nick »

How many steps has it got? They may have done like Pass Labs and only implemented one of the sets of resistors.
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#7 Re: 2P29L Preamp

Post by Ray P »

This is the Muses module I'm using in my buffered passive

https://edea75c2-1ee1-46a0-90b0-7171f55 ... 35bff7.pdf

and this is what I plan to use in the 2P29L preamp (I have the V1 boards and controller)

https://www.diyaudio.com/forums/swap-me ... olume.html
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#8 Re: 2P29L Preamp

Post by Nick »

Hard to know from the text, think there are four op amps, may be the devices on the back of the board. I have found the compensation cap on the input side op amps hasa huge effect on sound, check its at least a cog type used there.
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#9 Re: 2P29L Preamp

Post by Ray P »

I don't know what you're seeing on the back of the boards Nick?

Here's the manual for the module I'm planning to use in the 2P29L preamp.

https://onedrive.live.com/?authkey=%21A ... 45&o=OneUp
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#10 Re: 2P29L Preamp

Post by Nick »

I was only going from the first doc you sent, there were 4 devices on the back of the board. Yours may be different. I have no dog in this race.
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#11 Re: 2P29L Preamp

Post by Ray P »

Nick wrote: Wed Apr 14, 2021 10:03 am I have no dog in this race.
I know Nick.

I'll have a closer look again later after work.
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#12 Re: 2P29L Preamp

Post by Ray P »

Right, poked around the Academy Audio website and found some better pictures of the VCU board

https://www.academyaudio.com/product-pa ... trol-board

No opamps that I can see.
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#13 Re: 2P29L Preamp

Post by Nick »

I agree.
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#14 Re: 2P29L Preamp

Post by Paul Barker »

This sort of circuit bypasses my experience.

But 47 microfarad input caps warn me off.
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#15 Re: 2P29L Preamp

Post by Ray P »

Paul Barker wrote: Thu Apr 15, 2021 6:57 am But 47 microfarad input caps warn me off.
Paul, they're shown in the schematic as DC offset mitigation - also on the output if you look - it's typical for manufacturers to take this sort of cautious approach in datasheets. I have neither the input nor output caps in my buffered passive implementation as DC offset isn't an issue for me.
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