Hifi Preamps for your Record Player

By: skrodahl | October 16, 2018

Several people have been in touch lately with questions on how to modify the PCB into something it isn't right now. Popular requests are balanced inputs and/or balanced outputs, and DC servo on the outputs.

The PCB itself doesn't really lend itself to any of the modifications above, but you could always try doing add-on boards or roll your own from the open source freely available schematics.

An explanation of the Muffsy Phono Preamp PCB layout can come in handy for such alterations. And wouldn't you know, here it is:

Muffsy PP-4 PCB Layout
The Muffsy Phono Preamp PP-4 PCB Layout

Modifications you can do are changing the input impedance and altering the variable gain. I'm also happy to announce that the gain calculator has been updated to show the output voltage and voltage gain:

Muffsy PP-4 Gain Calculator
Muffsy PP-4 Gain Calculator

Finally, if you want to make major modifications, a way of simulating the circuit would really come in handy. Well, here's the Muffsy Phono Preamp LTSpice simulation file.

Muffsy LTSpice Simulation
Muffsy Phono Preamp LTSpice Simulation

By: skrodahl | March 15, 2017

The instructions give you three alternatives for powering the Muffsy Phono Kits. Here's another one:

 
Step-down power converter - Dual power supply
 
Step-down converter - Size comparison

This step-down power module is available on Tindie, and can provide up to two amperes of power. Use two of them and connect a 15V DC power supply at VIN/GND as shown in the picture above, and you will get a +/-12V output. This will provide sufficient headroom for the Muffsy Phono Kits.

The ripple is some 18-20 mV. This is waaaay higher than the Muffsy Power Supply, but should work reasonably well. Especially if you're strapped for space.

A small word of caution: The output voltage is set using very small surface mount resistors (included). You will have to do SMD soldering. It keeps the common ground though, so it should be able to work alongside other power supplies with all grounds connected.

No complaints on the price either, I paid $20 for five of these modules and international shipping is $3.

By: skrodahl | September 26, 2016

UPDATE:

Since only one extra resistor value is needed to get the higher impedances, it's been added to the kit. Now you can choose between these two options.

The input impedances on the Muffsy MC Head Amp can be changed to your liking. To help you find the right values, here's a nifty input impedance calculator.

Muffsy MC Head Amp - Default Input Impedances

You can play around with the values for R1, R2 and R3, and the calculator will show you what the input impedance choices will be. With all the switches set to OFF, the input impedance will be equal to R3.

The picture shows the values that are included in the kit.


PLEASE NOTE:
The resistors R3 MUST be installed, or you risk that the Muffsy MC Head Amp won't function.

Here's a suggestion for other component values, if you need a bit higher impedance:

Muffsy MC Head Amp - Alternative Input Impedances

By: skrodahl | May 20, 2016

NOTE:

This modification is only intended for the Muffsy Phono Preamp together with the Muffsy Power Supply and Back Panel in the recommended B0905 enclosure. Before you try this, have a look at the other ways of eliminating hum.


Arno just completed building the Muffsy Phono Preamp, and was haunted by distortion. He decided to use the B0905 enclosure for shielding and ended up with a completely quiet phono stage.

Using the B0905 cabinet for shielding

Notice also how Arno connected the ground lugs of the RCA connectors in a star pattern. Great idea Arno!

Using the B0905 cabinet for shielding, close-up

He drilled a hole in the cabinet and connected a piece of cable from the ground screw to this hole.


The reason this works is that the Muffsy Power Supply has a completely floating ground. If you're using a power supply that connects directly to mains power, this solution might inject more noise if not done correctly.

Arno's Muffsy Phono Preamp

You can see Arno's modification from a different angle in the above picture, and a larger version of his close-up below.

By: skrodahl | January 26, 2016

In Copenhagen, just across The Bridge from me, lives slowdiyer. He has designed the Muffsy Clone, his own version of the Muffsy that suits his needs exactly.


It is a mono version with single instead of dual op amps, which lets you use op amps like LME49710, LT1063, LT1028, AD797, OPA627 and more.  Being mono, you can use two power supplies and run it as dual mono if you'd like.


It does away with the variable input impedance, retains the variable gain and adds a mute circuit. The Muffsy Clone has space for a really large output capacitor as you can see in the picture below.


The form factor is 100x37 mm, compared to Muffsy's 84x56 mm. The boards are designed to be stackable.

The Muffsy Clone

There are a lot of other considerations that he's taken care of, all of which are described here. The same blog post also includes all of the project files, enabling you to make one for yourself.

Muffsy Clone in Enclosure

I'm anxiously awaiting his listening experience, as he's yet to finish the power supply to fire it up.

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