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Tube Phono Equaliser
“Hounddog”
Page 3, How to build a Dog
Last modified: 9 February, 2007
Building the Hounddog was relatively easy. Read this as: The power supply seemed easy, but the amplifier contained much more parts than I expected. And because I had the experience with Scratch that building tube amps too spacious invites noise and humm, I decided to build Hounddog with a minimum of space.
in the end, things that seemed so easy will always give you most trouble and take most time to resolve. In my case it was the power supply where transformers were not up to the task and power regulation was needed to get the filament power to work OK.
Although I can support most design ideas of Fred, I do think that he has attempted to sqeeze the last quarters out of some components in areas where these quarters do make a difference for better components or a slightly better setup. Moreover, I think the device will not work in the Netherlands at all without modifications (even if this is because of the 110-230 Volts power requirements). Therefore I made some modifications to the original design which I documented below and on the next page. For ease of understanding I assume you are familiar with the original design so you will understand the differences. If not, just build it the way I did.
The Original schema of Fred contains a provision for ceramic input. Well, for 10 years I have not seen such cartridges in serious equipment anymore and therefore I have omitted all components that were designed to provide this facility. It makes the feedback loop and the filter much simpler which is nice since I have little room available in the housing anyway.
First of all, the power supply. In Freds documentation two transformers are shown of 0.5A. The first is to transform mains power to 12V which after rectification is used for the filaments. But it also feeds the second transformer which is used is a step-up to get the B+ power for the tubes. The second transformer delivers 110Volts which is fed into a voltage tripler circuit. Since fred lives in the USA he used two identical cheap transformers of 12 Volts/0.5A. In my case, I had to use a 230/24 Volts transformer for the step-up to get the same results.
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Building and testing the power supply |
The secondary side of the transformer is according to specs 12Volts and 0.5A. For that reason I had chosen a transformer of 10VA which should be more than sufficient for my purpose. After all, Fred calculated that the power consumption of the amp was only 15mA at 430 Volts. This means that the transformer would use 6.5 VA only for the amplifier part which left 3.5VA for the filament supply. As I expect the heaters to need another 10VA, my conclusion was that Fred may have been too optimistic about his sizing. Therefore I recommend everybody trying to build the amp just like Fred did to oversize the first transformer to at least 16VA (1.3A) for 12 Volts.
Unfortunately I had already finished most of the power supply section when I dicovered this. And as a result I found that the power supply did not have the capacity to feed both heaters and B+. Therefore, I use an external 12V power supply to feed the step-up for the B+, and I must say: It worked withou much problems, but one thing remained: the power supply as described by Fred did not reach the 430Volts B+ by far. Hmm, I decided to have a look at this expecting that i made a wiring error somewhere in the design.
I started with checking all voltage levels and currents for heater and B+ circuit. At the input of the step-up transformer I measured only 0.2A (2.4VA) and at the heater supply about 0.55-0.60A measured after the rectifier circuit (about 7VA). Reading this text I still would assume that a 10VA transformer would be able to deliver the required power levels. However, it won't have any spare capacity and it will get very, very hot.
Therefore I checked all voltage levels again and I was NOT satisfied. The voltage levels as descibed by fred are not met. And although the design is not very critical w.r.t. the B+ voltage levels I decided to get a higher voltage on the step-up transformer in order to see the reaction.
Hey, that works. The B+ supply gets to the required level, and the sound quality rises at least with the same level. Also the feedback loop does work better, 10 seconds after switching from stand-by it is clear how noise and humm are almost completely gone. Hmm, this does bring me into problems, as 16Volts workt fine for the B+ but is not at all suited to get the heaters stable at 12.6 Volts. And apart from the heater problem, most capacitors are not designed to work above the 16 Volts (which reminds me to get 25Volts capacitors in my next projects.
Although it may increase the power consumption a bit, I decided that a voltage regulator IC would be probably just what we need in order to get independent from the exact voltage output of the main transformer. Such a voltage regulator, 78S12 etc. only cost one or two Euro/$ so it won't break the bank. And it will protect your heater supply, even if the rest of the circuitry is more or less independent from exact voltage levels. My 2 cts: Buy one that is capable of 2A regulation, additional costs are next to nill and it saves you money and room for a TO-220 which is $3 alone.
Apart from that, I built the heating section in an other way.
Making the heater supply symmetrical by grounding pin 9 is just too much for
me, so I did not do this. The heater pins 4 and 5 are connected to 12 Volts
supply and pin 9 remains empty. I did connect both 0/Gnd of both power supplies
with each other.
In the original design extensive use is made of Elco capacitors. Too much for my taste: for the output capacitor I wanted one of thes blue MKP of 1.0 uF/630V which are also used in my UL40-S2 tube amplifier. Total costs for two capacitors is less than €5.00. By the way, smaller values (until 0.33uF) should be OK for this application as well.
In the powersupply I used a higher value for the capacitor, a high-voltage
(450V) 220uF capacitor with a bracket which I kept in the drawer somewhere.
The mounting bracket makes it very easy to mount this huge capacitor to the
chassis.
During the construction where possible I made use of metalfilm resistors of 1 Watt (Beyslag) sold by conrad.nl on their website only (not in the catalog). These are heavy-duty resistors that do not break at the first instance when I made a mistake and need to bend things around or so.
Pending a full listening session with some friends, here a short impression of the sonical capabilities of hounddog after playing for two days. Remember that the tubes and all other components probably need more time to break-in, but the tubes (ECC81 and ECC83) already had some experience in the Scratch for some time and therefore I think we were in the area of 95% performance when listening after these two days...
During the first tests with the lower B+ Hounddog did work but there was not enough gain and there was some background noise present which was noticeable during playback (and therefore not really acceptable). The dog was clearly barking and needed come correction. With the higher B+ (400 Volts) noise and humm disappear within 10 seconds after switching from stand-by when the B+ reaches the required limit and the feedback circuitry kicks in.
This last point needs some attention: The hounddog needs some 10 seconds before the Voltage levels at the plates (and the Neon tubes) reach a sufficiently high level. When the Neons switch on, one can hear a clear tick over the speakers. Please remember to turn down the volume of the power amplifier or you will be sorry at some point in time.
The same aplies to switching off the smplifier. After switching to stand-by the humm level rises tot pretty high levels (probably because the feedback loop ceases to work) and at some point the Neon bulbs turn off. Again, it is probably wise to turn down volume of the amplifier or select a different source on the preamp while switching off Hounddog.
The results are nothing to be ashamed of (as far as sound quality is concerned). I feel really motivated to get rid of the external transformer and setup the power supply section. The noise floor is, as expected with the negative feedback, lower than than Scratch. In a first preliminary listening session Annemarie preferred the "other one" (=scratch) but the differences were very small and I'm convinced to bridge the gap for a great deal with some additional modifications. Every amp has it's own charm, and we start to love the Hounddog which maight result in a permanent family membership. OK, this project started as a study to active RIAA filtering but I hate to say goodbye to our pets and I'm finding something useful for the Hounddog to do. Hmmm, I will look for another Thorens TD16x turntable for a second permanent setup in the study so that I can listen to this amp as often as I like.
Is the Hounddog amp really finished now? Well, not really.
First of all, I want to get rid of the extra transformer by getting one new special tube-transformer of 16-24VA. Secondly, the prime transformer gets really hot (and I mean hot). Probably it won't hurt, but I decided that I won't take the chance, especially not since there is not much ventilation in the small housing.
My ideas are as follows:
Finally, I plan to use better tubes at some point in time. At the moment I'm using two Sovtek 12AX7-WA types and these tubes are not considered high-end by any standard. The new LPS types are considered to be much better for Hifi applications. I have ordered some ECC83 tubes from AE-Europe and these new JJ (formerly Tesla) tubes are much better quality than the Sovtek ones (red: I use them in the Tweety and Loekie projects)
© Maarten&Annemarie, 2001, 2002, 2006
