Muffsy MC Head Amp

Description

A kit for this project is available on Tindie:
https://www.tindie.com/products/skrodahl/muffsy-mc-head-amp-kit/

The Muffsy MC Head Amp is a dual mono active step-up preamp for LOMC cartridges. The purpose of a head amp is to allow low output MC cartridges to be used with a standard MM phono stage, such as the Muffsy Phono Preamp.

The Muffsy MC Head Amp has the same pitch black, silent background, the great sound stage and revealing sound quality that the Muffsy Phono Preamp is known for.

It uses an ultra-low noise op amp with a DC servo to amplify the LOMC's signal to an MM level. The input impedance is configurable from 47 to 100 ohms in 4 steps.

The PCB is designed to slide into a B0905 cabinet together with the Muffsy Dual Power Supply.

A back panel is also available, in order to make a fully working head amp without any

Details

Kit available:

Get it on Tindie!

Design goals:

Make a complete, nothing missing, DIY kit (the kit is now available, see http://muffsy.com)
Dual mono topology
Price target: USD 79
Easy and understandable build documentation
SNR above 70 dB (referenced to 0.5 mV input)
Variable input impedance
DC servo to eliminate input/output capacitors
Gain of 24.6 dB - 0.3 mV input gives 5 mV output (0.5 mV in: 8.5 mV out), at 1 kHz
No power cables across the signal paths
Dual layer PCB, immersion gold plated
PCB width: 84.0 mm / 3.3 in
Make the complete head amp with PSU fit in a B0905 enclosure
Improved back panel

Project Status:

Schematic, routing and PCB rendering for Muffsy MH-1 available here
Schematic: Done
PCB layout: Done
Component selection: Done
Component sourcing: Done
PCB manufacturing: Final version received
Testing/measurements: Click here for details, and see specifications below
Documentation: Done (these project pages), and here: http://www.muffsy.com/build-the-muffsy-mc-head-amp-mh-1.html
Kit ready: Done

Specifications:

Topology: Dual mono
Gain: 24.6 dB
Input impedance: 47 - 100, or 50 - 300 ohms in 4 steps, or choose your own values
THD: < 0.1 % *)
THD+N: < 0.1 % *) (measured: 0.6 %)
SNR: 71.8 dB
Frequency Response: +0 / -0.1 dB (20-20.000 Hz)
IMD (ITU-R): 0.001 %
Dynamic range (referenced to 0 dBV output): > 110 dB
Power requirements: +/- 15V at < 40 mA
PCB Size: 84.0 x 59.4 mm / 3.3 x 2.3 in

*) The inherent noise and harmonics in my signal generator is too high too accurately measure the THD and THD+N at these low levels. The numbers are too high, both should be well below 0.1% as the measurement without the noise shows that. I've added my measurements for reference anyway.

Files

Muffsy-MH1-Impedance.xlsx

Input Impedance Calculator

sheet - 38.06 kB - 09/28/2016 at 10:22

Download

Components

1 × PCB Complete kit available at http://muffsy.com

2 × 47 ohm Resistor 0.25W, 1% metal film

2 × 100 ohm Resistor 0.25W, 1% metal film

2 × 150 ohm Resistor 0.25W, 1% metal film

4 × 220 ohm Resistor 0.25W, 1% metal film

Project Logs

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Measurement Preamplifier
skrodahl • 10/10/2016 at 10:35 • 0 comments

The noise floor on my Quant Asylum QA400 audio analyzer is too high to measure the Muffsy MC Head Amp. Fortunately, Quant Asylum themselves have the solution, use a high quality preamp to lower the noise floor: http://www.quantasylum.com/content/Home/tabid/40/Post/205/Extending-the-QA400-Noise-Floor-with-a-Pre-amp

So... Looking at the Muffsy MC Head Amp, I realized that I do indeed have a high quality preamp sitting right in front of me. :)

I grabbed one of the prototype boards, one of my old PSUs, a back panel and an enclosure and started soldering, and there's the measurement preamplifier. It will be modified by using multiturn trimpots for variable gain, but there it is. I am now able to use the Muffsy MC Head Amp to measure the Muffsy MC Head Amp!
MC Head Amp - What, Why and How
skrodahl • 09/29/2016 at 12:53 • 0 comments

What is an MC Head Amp?
Why would you want one, and how do you connect it to your stereo?
This blog post describes it all: http://www.muffsy.com/blogs/post/Why-Would-You-Want-an-MC-Head-Amp/
Modify the Input Impedances
skrodahl • 09/19/2016 at 13:27 • 0 comments

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 the two.
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.
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:
If you decide to use the alternative input impedance scheme, the required components are included in the kit. It also includes a sticker that you can apply to the PCB with the new values.
The Final Version of the MC Head Amp Sings :)
skrodahl • 09/18/2016 at 20:42 • 0 comments

It's done, it's built, it's the first of its kind, it's plugged in, it sings!
UPDATE: The kit is here and available on Tindie.
No hiccups, no artifacts, it just works playing the 40th anniversary "Crime of the Century" by Supertramp.
Sweet :)
This does mean that, with some effort, the kit will be ready in about two weeks.
Also, the build instructions are very close to finished, I'm still writing the part about testing to make sure your head amp works before plugging it in.
http://www.muffsy.com/build-the-muffsy-mc-head-amp-mh-1.html
I Made it to the Hackaday Front Page!
skrodahl • 09/18/2016 at 11:13 • 0 comments

The Muffsy MC Head Amp made it to the Hackaday front page, that's really cool!
http://hackaday.com/2016/09/16/hackaday-prize-entry-high-end-preamps/
There's been a bit of a mix-up between this project and the Muffsy Phono Preamp though, so I wrote about it on the Muffsy Blog to clear up some of the misunderstandings:
http://www.muffsy.com/blogs/post/The-Muffsy-MC-Head-Amp-on-HACKADAY/
Assembling the Muffsy MC Head Amp Kit
skrodahl • 09/13/2016 at 21:30 • 0 comments

The assembly of the kit has started, first out are the cables, connectors, sockets and switches:
Muffsy MC Head Amp - Build Instructions
skrodahl • 09/11/2016 at 20:26 • 0 comments

The build instructions for the Muffsy MC Head Amp is now available on the Muffsy home page, still hidden from the site's menus as it's not yet available.
The instructions can be found here:
http://www.muffsy.com/build-the-muffsy-mc-head-amp-mh-1.html
The Final Version of Muffsy MC Head Amp Soldered
skrodahl • 09/11/2016 at 16:02 • 0 comments

Here it is. There are still a lot of components coming in, documentation and packaging needs to be completed as well.
Back Panel for the Muffsy MC Head Amp
skrodahl • 09/06/2016 at 14:33 • 0 comments

For those of you familiar with the Muffsy Phono Preamp, the connectors on the back panel have to be grounded (using cables) like so:
This is perfectly fine for the high output that an MM cartridge delivers, but it can be a source of noise for the lower MC signals. Here's a solution to that problem:
The RCA connectors and the ground screw will have direct contact with the copper rings around the mounting holes. These copper rings are connected with copper traces beneath the black coating, eliminating the need for separate ground cables.
Here's a look at the other side of the panel, which fits the B0905 cabinet perfectly:
Here it is with connectors, ground screw and power switch:
Final Version of the PCB - Received
skrodahl • 09/05/2016 at 11:35 • 0 comments

DHL came to visit, we're getting closer and closer to the final kit: