Another milestone achieved with dbx 231s

[mbhangui]

Today I did some changes. I fitted a R-Core trafo, Regulated DC supply board + 47k volume control + Pass B1 in the cabinet for passive preamp. Converted the input selector board to output selector board (by just interchanging input and output).

Fitted the Rod Elliot's Preamp board with built in LM317, LM337 power supply and a 12-0-12 torroidal trafo and a 4 pole double throw switch to bypass the preamp.

Earlier it was like this
Pass B1 with volume control + input selector --> DBX --> Rod Elliot's Preamp + output selector --> AMP

Now it is like this
input selector + Rod Elliot's preamp with preamp bypass switch --> DBX --> 47K volume control --> Pass B1 --> output selector switch --> AMP.
I also followed the grounding scheme suggested by Rod Elliot, which is lifting the ground using a 10 ohms resistor and a ceramic capacitor and a high current bridge rectifier https://sound-au.com/earthing.htm. Also followed star grounding for all my devices. Absolutely no hum.

Volume control + Pass B1 + output selector



Back panel of the above. The input is actually output and the 3 inputs are actually output to 3 different amps. This cabinet was made by the earlier diyaudiocracts when it was run by our forum member Bibin.


Rod Elliot's Preamp with input selector and preamp bypass switch. This cabinet was also done by Bibin from diyaudiocrafts.



Final image. In the process of soldering I accidentally blew hot air from my smd rework station on a new bedsheet. My bed is my usual workbench. I have been banned from using the bedroom for DIY activities. A raspberry PI with Kali reclocker and a I2S dac with low pass out is source1. The allo revolution dac connected to usbridge is source2. Souce 3 is output from another 12 source electronic switch to which I have connected things like chromecast audio, airport express, cd player out, etc. Source 4 is output from turntable preamp.

Plan for next week is to tidy up the wiring by putting them inside sleeves and holding the sleves to the cabinet body.

 

[Hari Iyer]

Earlier i too was using something similar for my preamplifier - AC relay based selector an JFET input preamplifier + active gain stage + discrete class A buffer at the output powered by a hefty LM 317 + LM337 power supply with huge filtering.

Now i graduated to analogue selector switch, passive preamplifier and zero powe supply and am very much satisfied to have gone this route.

 

[Yelamanchili manohar]

I've had a few preamps, mostly dacs with inbuilt analogue preamps ( AMR DP-777 and esoteric K-01 ). And one pure preamp a T+A P1260R. Both the esoteric and the T+A are considered reference level at their respective price points. Subsequently, having read srajan ebaens review on 6 moons ( it subsequently became his reference preamp then ), I bought a MFA classic passive preamp. These are TVC preamps, which stands for transformer volume coupling. So there are no resistors in the chain. To my ears, this is the most transparent preamp I've ever heard. There is no power supply or remote. But it sounds just pure, like listening to one's source directly. Of course, no gain circuit means, the speakers have to be on the efficient side, or the power amps have to dole out tons of power when used with inefficient speakers. So not an ideal fit for all use scenarios. But works a charm with my speakers. Plus, one big advantage, is that any EMI or RFI in the source chain, gets filtered out, and only the pure audio signal reaches the power amps. So it sounds super clean and pure. Call me an advocate for TVC passive preamps now

Disclosure: All the above were bought by me used. I could never afford any of them new

 

[Kannan]

Passive preamps are the best way to go as long as impedance matching with source and power amp is good. Otherwise there could be roll offs and shoddy bass.
This holds true for both transformer based (TVC) and resister based attenuators (Stepped Attenuators).
However modern day OP amplifiers are excellent. They offer minimal crosstalk, precise channel balance, remote control options, balanced circuitry and just enough gain to drive power amps.
I wish more DACs would employ good quality dual OP amps this so that one component and another set of cables can be removed from the sytem.

Speaker sensitity does not matter as long as the source outputs more than 1.5 volts, which all modern sources do.
With low sensitivity speakers one needs a more powerfull amp as detailed in the specs of the speakers.

 

[keith_correa]

Passive preamps are the best way to go as long as impedance matching with source and power amp is good. Otherwise there could be roll offs and shoddy bass.

Actually there should be a mismatch and not a match.

 

[Kannan]

Actually there should be a mismatch and not a match.

Naughty fellow making all.of us confucious. But you have a point contextually

 

[Hari Iyer]

Are we talking about actual impedance or characteristics impedance? If it's characteristics impedance then it should be a match