Tag: Gyrator

Short of sand

Firstly, my apologies. It’s been long overdue to get back on this blog. I receive several emails from people asking me to share more. I hope I will, when I get the time and energy to get back on hi-fi audio projects. I still work a lot on synthesisers which keeps me very busy when I catch a glimpse of free time from my hectic day life.

We’ve all been hit by the short of semiconductors. Yet, the HV precious parts are in shortage as well. Luckily some of the key parts like LDN150 and IXTP08N100D2 are back in stock, not sure for how long. These are key parts for building any CCS or hybrid mu-followers for audio circuits.

Continue reading “Short of sand”

801a preamp revisited (part I)

It’s been a long since I last posted here some project stuff. I’ve been busy with work and family. On my spare time, spent mostly listening to music, developing some synthesiser modules and playing music as well!

I missed ETF this year, sad to say. Work commitments made it impossible to  travel to France. Hope that’s not the case next year as am keen to get back. I’ve seen some pictures and feedback and seems it was a blast despite the COVID restrictions.

On my exchange with Rod Coleman regarding this post, I asked him to send me a pair of his latest DHT filament regulator (version 9) to test them on my 801a preamp with a cathode bias resistor (degenerated or un-bypassed):

Continue reading “801a preamp revisited (part I)”

01a Low Gain DHT preamp (finished)

It was a half-hour job to mod my Mule preamp by swapping out the UX-120 and introducing the 01a (CX-301a in fact), add the resistor divider to deliver low gain on this preamp:

01a mule

I set the valves at 115V/4mA. The Source Followers are running at 15mA and the resistor divider is actually 100KΩ/33KΩ (Kiwame) which is what I had at hand. It worked like a charm.

The 01a is driving my Slagle AVC into the 300B amplifier. I can say I can hear now the subtlety of the thoriated-tungsten filaments of the 01a with the extra level of gain I was hoping for. The amp sounds very dynamic at high level and with the clarity of the 01a.

01a mule

This is indeed a clear winner. I will leave this preamp on my system for a while to confirm further impressions. For now, I just simply love it.

300B/845 Amp: Driver Options

For more than 6 months I’ve been playing with a lot of satisfaction my 300B amplifier with the 47 driver. Since then, I’ve made 2 modifications:

  1. Major one was the introduction of my Open-Baffle (OB) speakers. These are the “Liionidas” based on the LiiAudio C-15 and W-15 drivers. I need to do a proper write-up of these, but have to admit their sound has taken my system to another level. They are simply superb. Detail and dynamics are beyond expectations. Nothing less to be expected from the designer (Leon Huijgen) :

Initial test of the Liionidas OB speakers

  1. Second modification was minor, I removed the Zobel network from the input LL7903 to minimise loading to the source. Also, after several tests I found that the impact in the HF resonant peak is minor as the source has some resistance (below 100Ω) which is beneficial. So the 300B amplifier looks like this now:

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01a Low Gain DHT preamp

How many times I’ve been asked “how do I lower the gain of the DHT preamp?” many builders love the DHT sound and yet don’t really need that gain. Of course when you use a transformer load, step down is the obvious choice. And it works really well when you invest in a very good quality OPT.

However, when you use DHT with high rp, generally most of the low current ones, you face a tough challenge with regard to frequency response.

With the hybrid mu-follower topology (aka gyrator) we have the problem that the gain is fixed to nearly mu.

I’m in love with the DHT sound and in a preamp a 01a and the 801a/VT-25 DHT Preamp Update preamp cannot be beaten in my view. They all deliver a gain of 8 with a hybrid mu-follower topology.

This can be overcome in the following way: Continue reading “01a Low Gain DHT preamp”

300B SE Amp: 47 Driver

Going DHT end to end

As previously mentioned, I played around with the 46 driver.  I love it sound and is a great valve. However, there were 2 reasons that pushed me to switch to the 47. Firstly, I wanted an extra tad of gain. Secondly, I have a nice stash of RCA 247 globe which measure extremely well. I’ve been reserving it for a 47 PP amp with local feedback (a la Pimm) and hopefully will get to in the future. Anyhow, the 47 in triode mode has a mu of about 8 which in combination with the SUT, gives me good gain to drive my 300B. After tweaking on the bench the stage for optimal swing and distortion performance, I ended up with the following circuit:

Continue reading “300B SE Amp: 47 Driver”

300B SE Amp: 46 Driver (Part II)

The 300B amplifier journey continues as planned. A bit lower than expected, however small steps being made. Recently I mentioned about the 46 driver.

I made some progress on the LL7903 step-up transformer which is wired on 1:8.

Continue reading “300B SE Amp: 46 Driver (Part II)”

UX-120 DHT Preamp into 300B

My friend Rob was right. The UX120 sound is unique. Perhaps is due to its cylindrical anode, thoriated-tungsten filament. Anyhow, he was also right that they pick hum and are extremely microphonic!

Well, not a candidate for a preamp indeed. However, with a low mu of over 3, it just added an extra gain kick into my 300B system:

The modification of the Mule was quick an easy. Fitted back in the UX4 sockets, adjusted the SiC array to 6V (5.8V drop) on filament bias. The hybrid mu-follower boards were adjusted down to 85V and 7-8mA. They are DC coupled into a pair of Source Follower boards biased at 15mA

Also I fitted the copper screen board on the back as I had in my Ba/Aa preamps. This works perfectly and when grounded there is no hum picked by the valve. No need to cover the valve at all.

The sound is amazingly good. Level of clarity and detail which is unique of an DHT. Very similar to the 01a sound in my view. Just great. I had to put the volume pot at the output as the microphonic noise is terrible. Yet, level is set to be usable (very usable) in my system. Love this sound indeed.

The 300B still is driven by the D3a. I will experiment with them in the future as have few things to do before changing them.

3 versions of the gyrator board

Three hybrid mu-follower (aka gyrator) board generations

From left to right: 1) Standard Rev08 PCB with full flexibility of FET and TH components. 2) Rev 1.0s board with SMD except Rmu, protection drain resistor and LED as well as space for any nice big PIO capacitor. 3) the smallest version of all, all SMD except Rmu, film cap and standard TO-220 top FET and multiturn trimmer.

Very happy with the results in the board development. It does take more time and precision (you will need a microscope) to work with the MELF resistors and the SMD components in general. However, it’s worth the trouble if you’re looking to reduce the footprint.