Iron and Sand

Over the last few months life has been very hectic for me. Between work travel and family, I’ve struggled to find time to work on any audio related stuff. You probably noticed this as have reduced significantly the amount of posts over the last 4-6 months.

This is sad, as have many things piled up for sharing. Either way, my focus has diverted slightly over the last few months. I decided to get back to music performance after so many years. This is a great decision, which I’m very happy about. When I thought about which instrument to play again, I immediately looked into my tenor saxophone – which I played for so many years. Unfortunately due to family commitments, I can only play during night time, so saxophone was ruled out. So, what should I do? Of course, my mind then got stuck on to my next big instrument preference: the Chapman Stick. When I was at university, I discovered this amazing instrument. I believe it was Tony Levin from King Crimson the first one I heard playing it. Anyhow, as I couldn’t either afford one and also there were probably none available in Argentina at that time, I decided I would build one myself. Luckily my dad was very experienced in woodworking and crafts in general. With his help we designed and build one, which sounded really good.

Fast forward more than 20 years, I ended up buying a new Chapman stick myself. An amazing instrument which is extremely flexible in terms of what you can create, tone, expression and range. I’m yet a beginner, but a happy one. I’m struggling to find the time though to practice, which is what you need. I will get there eventually.

As you can imagine, the return to music performance ignited a few projects on the diy audio side. First one was the construction of a Marshall clone PP amplifier. I will write about it in due course.

Second project turned into diy distortion stomp boxes. Who’d have thought so? I spent many years in my quest to reduce distortions and now I’m going on the reverse path!

I spent quite a lot of my spare time to try different Germanium PNP transistor in different fuzz and/or distortion stages. Amazing results as they do sound very nice. I made some PCBS and am yet to complete some final stomp boxes for them.

With the Locky curve tracer I traced tons of Russian germanium transistors. When I get the time I will publish the curves as am sure there are quite a few folks out there who would be interested in grabbing their hands on them. There aren’t many available so this is gold dust!

Let’s now return to DIY hi-fi audio now…

You’d probably ask yourself what is this whole thing of “Sand and Iron”. Well, as you know I play a lot with sand with valves. But I love iron, specially where it performs at it best. A year ago I concluded a series of experiments (which I believe I shared on DIYAudio) around headphone amps. I posted here as well on this topic. The outcome of my experiments was the 2P29L headphone amp which is shown below. An amazing circuit which I highly recommend you build if you like HP amps:

2P29L Headphone Amplifier

The circuit is very simple, yet there are some subtle things about it. The 2P29L was covered extensively here, but there are 3 things about this valve which took me down the path of selecting it for the HP amplifier:

1. Tone: this valve has an unique tone and detail as a DHT.
2. Lack of microphonic noise: this is paramount if you’re looking to implement a HP amp
3. Filament current is very low – just 140mA.

I breadboarded this amp after so many different tests and found it to sound amazing.

Worth spending the money in the Sowter 8665 transformer. It’s a marvellous piece of transformer designed for headphones. The resulting circuit performance is really good with very low distortion and nice harmonic profile. I love this HP amp. I yet have to build this in a nice case.

Surely you’d ask yourself, can I build this without the output transformer? You can, but you will need to modify the hybrid mu-follower to balance current into the 32 ohm load. Also the 2P29L will struggle to provide the current needed. Even for just 100mW you will need about 60mA. You would need to have a PSE arrangement. Also distortion will be unavoidably higher. Yet, it’s possible.

What I ended up doing was to replace the cathode filament resistor for an arrangement of a SiC diodes and a small resistor for filament bias. Why? Simply because it sounds better. My SiC diode PCB holds only 6 diodes and to get 7V, I added a pair of 15R in parallel to deliver the extra volt needed:

The value of C2 is critical to provide a smooth frequency response and will depend on the transformer used and wiring configuration. If you have a 300 ohm headphones you will have to adjust the value of C2. I used Russian PIO ones which sound great. Any other boutique capacitor of your preference would work as well here.

For the 10Y/VT-25 fanatics you can easily turn your preamp stage into a similar HP amp:

The VT-25 can be microphonic enough for a HP amp. You should take enough measurements to tame this noise. The filament bias array made up of the SiC diodes will get pretty hot. They can withstand without heatsink with about 1.2W on each diode. You will need to allow for good ventilation. Otherwise you can bolt them to the chassis as the plastic TO220 case is very convenient.

Recently I have some queries about using the VT25/10Y preamp stage to drive an output stage and ways of eliminating a previous DHT stage. I did this long time ago with my 814SE Amplifier which has the crazy idea of a 46 driver with filament bias (yes it was my heating system) and an input step up transformer.

Here are two ways in which you can use the 801a/10Y/VT-25 in a driver configuration with a SUT. First example below shows a SUT made up with an interstate transformer:

The SUT gain may be limited here and you need the right OPT which can deliver lots of volts at very low distortion. With the parafeed arrangement though, the OPT can be smaller and easier to build to provide very good frequency response. The VT25 will carry the heavy load of the output stage grid capacitance which is multiplied by the SUT configuration. This arrangement also facilitates the negative grid bias of the output stage, which is quite practical as it has been done for ages. Limitation here will be on A2 performance though.

What I’d do instead is this second example. The SUT is located at the input stage. In this way the source will be challenged by the Miller capacitance of the VT-25 valve multiplied by the SUT effect. You should take this into consideration or otherwise you will risk of loosing some treble if the source + volume control cannot handle this:

The SUT can be easily 1:5. There are good Lundahl examples as well as the ones made up by my friend Dorin. I posted some measurements of his transformers before.

In this way, the output stage can operate to its best given the help provided by the source follower. If you are planning to use a transmitting valve like 845, 211, etc. This is mandatory.

Hope you find these examples useful.