Page 3: Construction

Last update: March 12, 2007

The finished Martian Amp.

Prototype 1

Well, this how it started, from the drawing board to a piece of wood.

Here you find a picture of the prototype Martin amplifier. I built the amp on a piece of wood and used an aluminum L-profile to mount the tube sockets, potmeter (right) and jack connector (left). I used a few cheap transformers, the B+ generated by a step-up transformer. Some important things I considered when building this amp, especially making sure I do not shorten its life are as follows.

• The nominal plate current should be somewhere around 80mA. But in order to extend the life of the tube a little I used 65mA per tube as measured for the prototype.
• For the heater supply I used 12Vdc with a power regulator PER side. This is lower than the nominal value of 12.6V but since it is well within specs and will extend the life of these tubes I will use the lower value.
• At first I used a step-up transformer for the B+ voltage. However, since I discovered that the amp consumes more power than I anticipated (and step-up being less efficient than expected) I now use a separate dual 115Vac transformer of Amplimo. These are not yet shown in the pictures (need to make new pictures of the final prototype) but the circuit shows the latest version.

The above considerations may seem to be nonsense, but on the web there are several sources telling me that the 832A is not a very strong tube and it's life expectancy is rather short. Well, I've 7 of these tubes but still I don't want to use all 3 sets within a short timeframe.

I built the following circuit on a piece of wood in order to be able to easily swap components etc. (click on the figure to enlarge it).

The amp worked immediately and the sound is not bad at all also. However, some resistors became very very hot and there is a slight oscillation, but that is probably due to the free cabling technique and the lack of housing.

 

 

 

Proto 2

 

At first built the whole amp on a piece of wood, including a rather primitive power supply. It consists of a 12V/60VA transformer for Halogen lamps, a step up transformer (12-220V), 4 diodes 1N4007 and a capacitor. Strange enough, this works quite well for both output tubes, but the driver stage probably needs better filtering. But for the moment an additional RC filter will do.

A little later, after hearing the amp was quite good sounding I decided to "invest" in better transformers. 2 transformers, one for heater supply (2x15Vac, 160VA) and another for the plates (2*115Vac, 120VA). Both are oversized (I know) but this will at least ensure I have enough power in reserve.

Also as you can see I changed the heater circuit a little and made better cooling using an aluminum cooler from an old computer CPU. In the final version I decided to use it too.

Above picture shows that I switches the caps on the output from 100uF Philips/BC types to 333uF/Rubicon. I like these Rubicon caps, but must say the difference is nil as far as I can see uhh hear. I will see what value will offer the required 5Hz point which I want in order to produce the lowest frequencies.

Final Enclosure

Knowing that I know that the amp works and that it sound good, it was time to look for a nice enclosure large enough to accommodate amp and the power supply. With respect to the last one I underestimated the whole power supply thing big way.

Resistors were running much hotter than I expected, and also the power supply initially did not nearly provide enough power for the amp. Filtering is not easy as both 832A tubes consume a lot of power, but given the architecture of the output stage (CCS) not much filtering is needed anyway.

Here are a few initial pictures of the final version:

The drawback of these enclosures is that although they are advertised as being aluminum the top and bottom plate are made of steel. Therefore they are sensitive for humm caused by transformers and as such I would not use them in a phono project. However, with toroids there is little chance on humm.

On the other hand, the advantage of these enclosures is that the front plate is not fixed to the rest of the chassis apart from 4 bolts and therefore the frontplate which can become very hot will not transfer it's heat to the front plate. The front plate will therefore remain relatively cool.

As I wanted to limit the risk of humm caused by transformers cramped inside the enclosure I decided to put the transformers on the top plate As I result I decided to but a new transformer for the heaters, switching from 160VA to 90VA. As you will see in later pictures of the amp I made a mistake here: Using two transformers with identical power would have yielded a better result cosmetically. Maybe you would ask why not buy a 2*115V transformer of a lower power rating? Well, this is because the next lower version of Amplimo for this voltage is a 50VA version which is sized correctly on paper but in practice would have little power in reserve.

Modification 1

The first modification involved the following changes to the amp:

• Change in the voltage regulation circuits. I used a simple voltage regulator board from Conrad for the heaters of each tube. Since the heater requires less than 1A each, I thought I was safe here. But the regulator prints of Conrad use 1N4007 diodes and it appeared that especially when starting the heater supply the inrush current is much higher. After a lot of smoke and 4 destroyed diodes I decided that this solution was inherently unsafe. As a consequence I changed the heater supply completely.
  The change involves using a large bridge rectifier for the two 832A tubes, and two separate regulator boards for each tube one. The regulator boards use 78S12 voltage regulators of 2A each and so we have some headroom there as well.
• Finding transformer caps. The two transformers (heater supply and B+) are mounted on the top plate of the amp and are standing naked and unprotected. Not that I'm worried to much about their health but on mine and other people close to the amplifier.
  I bought two small pots from a drugstore, used for cotton wadding. As you can see on the pictures: I used the lid under the transformers and used the pot itself upside down to put it over the two transformers.
• I changed some resistors in the amp, Most values were 5W and although that looks good on paper, temperatures in the enclosure did rise to an unacceptable level in my opinion and I changed these resistors to 11W types of put two higher values in parallel to split the current (and heat).

The result looks great and sound probably even better:

Output Impedance

Measuring the amplifier yields a Ri (internal resistance) of about 5k7 Ohms. Since we know that S=3.5, we can calculate the amplification factor u by the following formula: u = (Ri * S)/1000 = 20. We therefore can calculate the output impedance as follows:
Ro = Ri / u = 272 Ohms (per tube). The total output impedance therefore is halved and is approx. 136 Ohms.

Feedback

I will consider using feedback in a 3rd update of the amp. On the other hand, transforming the first stage to a SRPP will also provide current feedback and reduce distortion.

 

Parts List, Bill of Material

The part list for the Martian amplifier is found below (if time permits). Note: I will look up as many as I can but some costs will be approximations based on my memory.

		Amplifier
#	Part No	Item Description	Price
2	V2	Tubes 832A	$ 2.00
2	V1	Tubes 12AT7	$ 10.00
2	C1	Auricap capacitors 1 uF	Eur 10.00 a piece
2	C2	BC Components, Elco 470uF/50 V
2	C3	Rubicon 220uF/400V	Eur 8.50 a piece
2	C4	BC Components 10uF/400V
1	R1	ALPS Blue Pot 50k log	Eur 12.50
2	R2	470 Ohm, 1W Beyschlag
2	R3	47k Ohm, 1W Beyschlag
2	R4	1k Ohm, 1W Beyschlag
2	R5	330k Ohm, 1W Beyschlag
2	R6	2k2 Ohm, 5W Wirewound
2	R7	100 Ohm, 1W Beyschlag
2	R8	390 Ohm, 1W Beyschlag
2	R9	100 Ohm, 1W Beyschlag
2	R10	220 Ohm, 5W Wirewound
2	R11	470 Ohm, 5W Wirewound
2	R12	22k Ohm, 1W Beyschlag

 

		Power Supply Unit
1	TR1	Transformer, Pri 230V, Sec 2x115V, 120VA (http://www.amplimo.nl no. 48070)	Eur 32.00
4	D1-D4	Diodes 1N4007	Eur 0.30 a piece
4	C8-C11	Capacitors Wima MKP 0.022uF/1000V (one parallel to each diode)	Eur 1.00 a piece
3	C5,C6,C7	Capacitor 330uF, 400V (Rubicon)	Eur 9.00 a piece
2	R13, R14	Resistors 680 Ohms/5Watt Wirewound
		Heaters Supply 832A
1	TR2	Transformer, Primary 230V, Sec 2x15V, 80VA (Amplimo no.38013)	Eur 28.00
1		Rectifier Bridge (Min 4A)	Eur 4.00
2	VR1, VR2	78S12 Voltage Regulators (one per 832A)	Eur 1.00
4		Elco 2200 uF/25V
		Heaters Supply 12AT7
1		Voltage Regulator board (Conrad) complete kit without Voltage Regulator	Eur 5.00
1	VR3	Voltage Regulator 78S12	Eur 1.00
		Misc.
1		Enclosure	Eur 26.00 ???
1		Volume Knop	Eur 7.00
2		RCA connectors (input)	Eur 4.50 a piece
1		Headphone connector (output)	Eur 3.50
2		Tube feet noval/9-pin
2		Tube feet Septar/7-pin for 832A	Eur 4.50 a piece
4		Standoffs for soldering resistors and wires close to the tubes
2		2 meters PTFE tubing for insulating resistor and other bare wires
1		IEC power inlet with mains filter	Eur 3.50
2		Fuse holders 20mm fuses
1		Fuse of 600mA, Slow (T)
1		Fuse of 800mA, Slow (T)
1		Cooler for the two voltage regulator (old PC cpu cooler)	Eur 0.50
2		Switches for heater supply and for plate voltage (B+)	Eur 2.50 each
2		2 pieces 5mm LED for under 832A
2		2 pieces 3mm LED for under 12AT7

 

Links

• The 832 amplifier of a Malaysian DIY audiohobbyist
• Ultranalog probably built the first version of a 832 amp, the Coldwar
• The Distortion Cancellation circuit of Illuwatar
• TubeCAD article on Inversed Distortion Cancellation
• My Sennheiser Headphones

 

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© Maarten&Annemarie, January 2007

last modified 8 January, 2010