My friend Bela, who is an experienced DHT user and very fond of the 801a, asked me if I could help in developing an LTSpice model for the venerable 801a. I have several 10/10Y but unfortunately no 801a at hand to trace so I used the data sheet curves which are a bit challenging due to resolution/scaling of the characteristic curves:
801a model from data sheet curves
I had to compromise the model fit as if you want accuracy in A1 region the matching is less than perfect in A2. Still, the model is not perfect but is very good for simulation purposes.
Great pleasure this morning to receive the first batch of the gyrator PCB. After extensive testing we refined the layout and options for the PCB and now we have the final product ready for shipping! Unfortunately I will not be able to do some testing and shipping before end of February and I received a large number of requests already. I’m sure this batch will fly very quickly so please confirm your requests.
I will post soon the specifications and some circuit examples for this flexible gyrator board which can be used for DHT preamps (e.g. 4P1L, 01a, 26), amplifier drivers, A2 drivers, LTP drivers, parafeed output stages and more!
So check the For Sale section soon for more information.
Some days ago we discussed in DIYAudio using this board for a 2J27L preamp like this one:
2J27L Preamp using Gyrator PCB
The output FET follower is needed to for valves which have low current and high anode resistance which will struggle to drive large capacitances. The FET follower of your choice can be used instead.
A minimal circuit which sounds fantastic is the basic configuration of this PCB. With few changes this circuit can be used on many DHTs like 26, 4P1L, 10Y, 30, 30sp, 12, 71a, 45, 46, etc:
If you want to send me your requests please use ONLY the form below:
Some of you may be a bit fed up already with these slew rate posts, however I find this fascinating as is taking me through different routes of experimentation.
On my last tests, I abused the DN2540 to an extent which meant the dead of it. So I ended up adding the appropriate back to back protection zeners on the gate:
Are you writing again about the “Slew” thing? There is a lot to be said about the slew rate still. In fact, this post is about some real life measurements and experiences had in the past few days.
This week I went to my friend’s (Tony) place to listen to his 01a with the additional follower discussed previously. Tony complained about that the addition of the follower changed the sound of the 01a. Something was lost, detail on the treble, airiness. I brought up again my test gear and surprisingly we found some H3 level which wasn’t there on the previous preamp. Odd, could that be the FET follower? In the end after several tests, I checked the operating point (you should never assume they should be as expected) and found that the follower wasn’t operating at the optimal level, source current was low and also the 01a was running out of place. I recalibrated the gyrator and voila. The sound of the 01a, came back!
Unfortunately we ran out of time and I did not take any measurements again after re-calibration.
It was time to submit the power supply to a quick test:
I used a 1:1 transformer to measure the supply noise using this jig I built time ago. The test load was a simple pair of 4K7Ω 50W clad resistors. All wiring was done with alligator clips so induced hum was obviously picked up.
The 50Hz artefact is induced noise from the chokes and circuit being close enough and no shielding was in place for any of the measurement cables. The 100Hz and other byproducts of the rectification is really low below 3mV The overall inband noise is just above 50mV which is mainly the induced noise of the supply being so close to the test load.
There’s not much information around the 6Ж49П-ДР or 6Z49P-DR / 6J49P-DR pentode. This pentode was designed for high-frequency operation and durability. It has relatively high transconductance and high mu when is triode connected. Hence, an interesting device for a phono stage given the low capacitances in place as well. The 6Z49P is close to his brothers 6Z51P and the famous 6Z52P albeit it has different characteristics.
Here are some picture of my slow progress on this flexible power supply. I started with the layout of the parts before any marks were made on the top plates:
Working out the layout of the top plates
Once I was happy with the layout, I submitted myself to the mercy of the most dreadful job on earth which is metalwork!
When I was still a teenager and learning saxophone I came across this wonderful version of “Get thy bearings” by King Crimson. The power of the electrified alto sax blew me out my mind. Ian McDonald clearly found a way to drive the saxophone there and bring a new sound to the 1969’s progressive rock. Surely he didn’t experience any of this slew rate thing 🙂
In my previous post, I explored the slew rate challenge of the 01a preamp:
You may probably already asked yourself: what is this “slew rate” business? Has this guy lost his mind? Probably I have, but not due to this interesting phenomenon. The second question that should have probably popped in your brain is: why? Well, writing about this stuff came up by sheer coincidence.
We tested my friend Tony’s 01a preamp which has an older gyrator board I made for him about 3 or 4 years ago. The preamp (as well as his system) sounds extremely good in my opinion and the 01a has brought a new clarity which is what you’d expect as a result of the introduction of a DHT stage. We decided to run some frequency response tests since Tony has made some interesting mods to his Push-Pull amp. We encountered an unexpected challenge as I didn’t have the right XLR connectors for my testing gear so we run the FR tests on the power amp with the 01a.
The 01a preamp measured as expected with a flat response up until 40kHz before the sound card rolloff kicked in – I’m aware of this and this is the limited bandwidth of my current portable measurement gear. The distortion was also really low (H2 predominantly ) with less than 0.02% for 2 Vrms output signal. This test was done with Pete Millett’s interface which has an input impedance of 100kΩ: Continue reading “Slew Rate in Preamps”
