Hoekstra DC-Coupled 300B ... oooooh!
 

I just stumbled across what looks to be a fantastic SE circuit design by Bob Hoekstra, called the Axiom, which I've decided I probably will build, and which I thought I'd post here for the benefit of others. It's a DC-coupled 300B design that sports stacked power supplies. I first learned about stacking supplies by viewing the DCMB circuit, which I posted here earlier. The stacked supply concept rids a circuit of nasty cathode resistors (YAYYY!!) and allows direct coupling (YAYYY!!). The Axiom design differs from the DCMB implementation by using inductive rather than resistive anode loading---another YAYYY in my books.

Another interesting element of the Axiom design is the use of damper diodes in the cathode circuits of all tubes, and in the anode of the 300B output. The diodes, from what I can tell, simulate to some degree a constant current source, giving a measure of power supply regulation.

Interestingly, I stumbled on the Axiom amplifier by researching shunt regulated power supplies. The Axiom is posted on Chimera's website (pick me), the company being the originator of the Kyrie 211 amp that sported .... a 211 shunt regulator. Well, the Axiom seems to better my idea of same-tube shunt regulation by a simple design implementing a three-stage amplification circuit using (COUNT THEM!) 28 parts and 1 resistor (I'd replace the input volume pot with a transformer)!

The amp's distortion specs, measured using an HP FFT Dynamic Signal Analyzer, are a sight to behold:

1 Watt = THD 0.042%
2 Watts = THD 0.110%
5 Watts = THD 0.512%
6 Watts = THD 1%
7 Watts = THD 5%

Smoking!

The schematic can be found here .

hi there

I saw this design a year or so ago, and thought that it was an awfully complicated way to go to avoid using resistors!

the diodes in the cathodes of the tubes are to set the bias points. they don't have a constant current effect.

they effectively lift the cathode, so that the grid is more negative that's all.

some people use LED's in the front end of amps for much the same purpose.

kind regards

bill

Dennis Boyle, who I guess is developing this amp as a kit, suggests on his website that the diodes reduce the amp's overall distortion (measured). Does the reduction result from dynamic vs static resistance of the diodes? Here is are distortion comparisons, and subjective impressions, from the Chimera website:

Following are some additional test results that will help put the Axiom’s design into perspective.

All of the diodes were removed and equivalent resistors substituted.

1 Watt – THD 0.557%

Compared to the traditional design the Axiom did well even without the diodes. The following tests were done sequentially with resistors being removed and diodes inserted.

The resistor in the cathode circuit of the 300B was replaced with a diode.

1 Watt – THD 0.123%

The resistor in the cathode of the 6J5 was replaced with a diode.

1 Watt – THD 0.079%

The diode was added to the 300B’s anode circuit (with the previous diodes still in the circuit)

1 Watt – THD 0.051%

The resistor in the cathode circuit of the 6Y6 was replaced with a diode.

1 Watt – THD 0.042%

Blind listening tests for the various iterations were as follows:

Diode in the Cathode of the 300B
Significant improvement in presence and low level detail

Diode in the Cathode of the 6J5
Better dynamics and a slight improvement in low level detail

Diode in the Anode of the 300B
Improved open-ness and slight improvement in low level detail

Diode in the Cathode of the 6Y6
Slight improvement in low level detail

If the diodes merely set bias, I wonder if increasing the DCR of the anode chokes (and corresponding inductance) and ridding the circuit entirely of diodes would improve performance?

Anyone have any idea why diodes in this circuit would have the effects suggested in the quote above?

Hi,

I seriously doubt it would, you want high AC impedance not high DCR.
So, yes, maybe a bigger choke might have been better but I assume these guys know what they're doing and already arrived at a good compromise between high inductance and linearity.

Equivalent in what way? A diode doesn't behave like a resistor would, if it would the result would amount to the same for either of them.

The way I see it, the diode basically acts like a bypassed resistor without the non-linearities of the bypass cap.
From the description given in the listening tests I recognise most of what I hear when I ommit the cathode bypass cap. The difference here is that you retain the advantages of the AC short the cap provides.

I haven't read the pages on the amp yet so, maybe there's more to it than just that....

Cheers,;)

serengetiplains

If the amp suites your fancy then by all means build it. Keep in mind however that distortion specifications aren't everything. The proof is the final listening experience.


Joe

Hi Frank, I was suggestin increasing choke inductor DCR to increase DC voltage drop across the choke, which increase, without diodes in the circuit, is necessary to maintain proper bias voltage for the next stage (that voltage drop is the bias voltage). If the diode acts as a bypassed cathode resistor, there seems very little difference between this circuit and the DCMB circuit I posted in a previous thread. The main difference is that resistor anode loading is scrapped for choke loading, which I prefer.

The idea of series diodes was also used in Hiroshi Uda's STC amplifier designs where they were referred to as 'stopping diodes'. Not sure how relevant this is to the circuit under discussion as they were connected in the ht side of the output transformer, but maybe worth a look anyway.

schematic here: http://www.audiofanatic.it/Schemi/Ti...5GA_SE_STC.jpg

discussion here: http://www.alphalink.com.au/~cambie/STCNotes.htm

"(1) Effect of the SD
Inserting a diode between the B+ terminal of the output transformer and the power supply itself, in the typical STC amplifier, "stiffens" the sound of low frequency music content.

This phenomenon implies the existence of some inter-relationship between this type of amplifier and the loudspeaker system. The SD becomes more effective when a small output transformer is used, rather than a large one.

The effect of the SD is not merely specific to the STC amplifier however. It is also effective in other non-NFB or local-NFB amplifiers. In the case of overall loop NFB amplifiers, the effect does not appear at all or is reduced. Supposedly, the reason is that a loop-NFB amplifier acts to amend the shape of signal waveforms passing through, by the NFB action."

Re: Hoekstra DC-Coupled 300B ... oooooh!
 

Konnichiwa,

Actually, the diodes operate as a distortion cancellation circuit, by "pre-distorting" the Valves drive signal (between grid & cathode) such that the following valves distortion is complementary. However, the ONLY distortion eliminated thus is even order, just as in Push-Pull amplifers and the cancellation depends on many factors.

Past that, no, the diodes are not operating as current source, they operate as non-linear resistor.

Sayonara

Diode distortion
 

Thanks for the insight. By a form of slow, ponderous logic, I was heading the direction of that insight as I noticed from the tube curve of the diode that it operates in a non-linear fashion, and the more so, seeemingly, the lower the current draw.

If by this predistorting feedback only even order harmonics are cancelled, are odd order harmonics thereby increased? This is something I've never entirely understood.

Re: Diode distortion
 

Konnichiwa,

Please note, there is NO feedback in that sense.

As to odd vs even, if you use any methode (*) of cancelling even harmonics the invariable result is an increase of odd order harmonics and an increase of the number of harmonics. That's the way of life.

Sayonara