DIY preamp

[Art Vandelay]

Thought I would share this.

See pics attached that show the various measurements of distortion and square wave performance.

A few points first though:

1. The preamp is a solid state design with a current mirror loaded LTP input stage, single ended vas and single ended output stage.

2. Global feedback is only about 13dB @ 1kHz, falling to zero dB at about 300kHz. Mostly local feedback is used to reduce distortion.

3. Output stage bias is nominally 100mA (high) but varies (slightly) to maintain a constant heatsink temperature of 40C.

4. Linear Systems bipolar (input and mirror) transistors are both dual package matched devices - for lowest distortion, thermal tracking and low offset voltage.

5. A combination of discrete series and shunt regulation used throughout, and the linear supply is also choke regulated - and located in a separate enclosure.

Distortion fft plots

Distortion testing was done at 1vrms, but is not amplitude dependent.
The test frequency is 6khz rather than 1khz - to better stress the device under test.

The first chart shows the loopback test which is the performance of the sound card itself.

The second chart shows distortion of the preamp into a 47k impedance

The third chart shows distortion of the preamp driving a very low 100 ohm load.

Square wave testing

This is used to test stability and bandwidth

1st pic @ 1kHz
2nd pic @ 10kHz
3rd pic @ 20khz
4th pic @ 50 kHz
5th pic @ 100 kHz
6th pic @ 200kHz

[jdandy]

Art.......Impressive. The square wave performance out to 100kHz is exceptional. It would be interesting to see the first four tests with a 1kHz signal, a more typical test frequency for audio square performance. Thanks for the photos.

[Art Vandelay]

Originally Posted by jdandy

Art.......Impressive. The square wave performance out to 100kHz is exceptional. It would be interesting to see the first four tests with a 1kHz signal, a more typical test frequency for audio square performance. Thanks for the photos.

Thanks for the comments.

When I get some free time later I'll run the test at 1kHz, plus I'll test my Krell preamp so that you have a reference.

I'm slightly limited by the sound card, as good as it is (Asus xonar), but it appears as though all distortion products are at or below 0.001%, across the audio band.

I might also run the thd tests again with global feedback reduced to 0dB, which I can do easily enough by changing a couple of resistor values.

In terms of subjective performance, I prefer it over my three other preamps.
Soundstaging is excellent and the treble is very pure and sweet, and free from grain, but very detailed at the same time. Everything else is beyond reproach too.

With 30dB of global feedback the treble was slightly less liquid, and transients took on a slightly too hard quality, but around 10dB seems optimum.

There is a DC offset controller which thermally tracks the preamp's drift, but it's outside the feedback loop.

Btw, the -3dB point is 1.7MHz, set by the input filter.

[Cohibaman]

I'd love to know more about the design & build and see some better pics with the top off.

[jdandy]

Art.......Is this preamplifier a kit that you are tinkering with or one you designed and built from concept to finished product?

[Art Vandelay]

Originally Posted by jdandy

Art.......Is this preamplifier a kit that you are tinkering with or one you designed and built from concept to finished product?

The latter.

I'm out atm, but I'll reveal more later when I arrive back home.

It's actually a prototype for a commercial project that I've been hired to do. The end product will look very different but will include most of the features, and maybe more.

This is a single ended pre, but I'm also working on a fully balanced design too.

Art.

[Art Vandelay]

I've taken fft + square wave measurements as before with a Marantz SC11S1 preamp and a Krell KSP-7B preamp.

Both are mid priced preamps. The Marantz is designed to be a low distortion preamp - using HDAM modules and it's a fairly new design.

The Krell is old school from the late 90's, using a jfet input stage and high bias to drive low impedances.

Note that I was unable to perform the 100khz square wave test on the krell, because it wanted to oscillate.

[Art Vandelay]

To summarize above, the Marantz performed much better than I was expecting, although it wasn't quite up to driving the demanding low impedance.

The Krell's distortion was much higher but predominantly low order, but still no worse than 0.01% which is quite acceptable, and it drove the demanding 100 ohms without producing any additional distortion.

The Krell performed below par above 20kHz with the square wave tests, and oscillations > 80Khz indicated stability or slew limiting issues, but it's worth pointing out that this isn't a real world condition so the preamp works just fine with normal bandwidth limited audio inputs.