I realized today that it's been nearly a year since this blog has been updated, but that doesn't mean that Wombat Amps hasn't been busy! On the contrary, we've now launched an improved web site, I've established a formal Wombat Amps workspace in a building that also houses dozens of bands -- as a consequence I've been doing a fair amount of repair work -- more than I had really intended actually. Nevertheless, I have been working on some interesting projects, including a cool custom amp that was recently completed and a solid state equalizer.. But details of these just haven't made it to the blog for want of time.
Nov 14, 2013 Attached is the result of my 351 line output transformer tests. According to the Ampex 351 schematic, each half of the 12AU7 driving the transformer is drawing 6.5mA and has a plate-to-cathode voltage of about 275V. Looking at the 12AU7 data sheet, this gives an Rp of just about 10K, hence the 10K resistors driving the transformer. Ampex 440/1100/1200 series studio recorders. The original Ampex head amplifier design has been combined with a new modern head input transformer. The line amplifier stage has been completely redesigned to provide improved specifications and sonic performance. The PCB’s are full ground plane types for improved shielding and circuit stability.
However, I'm working on a new microphone preamplifier project that I'm really excited about, and I hope to document a fairly complete build log here. This mic preamp was commissioned by Andrew Everding, perhaps best known as the keyboard player for the band Thursday. However, with Thursday currently on hiatus, Andrew has moved on to audio engineering, and is currently located in Auckland, New Zealand, where is an audio engineer working directly with Neil Finn (of Crowded House fame) at Neil's well-known Roundhead Studios.
In any case, Andrew is a big fan of tube microphone preamps based on modification of old Ampex reel-to-reel players. As a tube aficionado, this makes perfect sense to me - in a world now flooded with sterile-sounding ultra-low distortion preamp choices, more and more musicians are realizing that a bit of distortion, of the right kind, can be just the ticket for producing a warm, full sound.
One of the preferred platforms for the mod community has been the amplifier from Ampex 351 reel-to-reel player, as shown above. In fact, there's a fair amount of documentation online (especially at the Electric Audio forum, courtesy of Greg Norman), describing how to mod one of these units into a preamp. So why not just mod one of these, and be done? Well, it turns out that there are a number of good reasons. First, the Ampex 351 is now highly sought after, and prices for these units have skyrocketed. You can easily pay $1300 or more for one on fleaBay, and what you get will be in uncertain condition. And this is before the mod process, which turns out to be not at all straightforward. These were constructed on now crufty old first-generation PCBs that are prone to traces lifting from accumulated moisture, and are not particularly amenable to modification. Also, there is a ton of other circuitry in there that gets in the way and is unneeded after the mod, you're for sure going to have to replace all the power supply capacitors anyway, etc., etc.. Even after you go through all the trouble, you're still going to be left with just a single preamp, not a dual. So you'd need to do this all twice to get a dual setup! And this single would not really be optimised in terms of layout, ground scheme, noise,
etc. It also wouldn't provide modern amenities such as phantom power and fully regulated DC tube heaters, at least not without having to do a bunch of extra work. But aside from all that, the Ampex is still a pretty damn good reel-to-reel player, and I just Ampex 351 Input Transformer Fuse
hate seeing beautiful old gear being canabilized in that way. So the decision was made early on in the process to scratch-build from new components a dual preamp that stays true to its heritage but incorporates just the components necessary for a preamp. In addition, it should incorporate features such as phantom power and regulated DC tube heater supply, and do it all in an attractive 3U rack-mountable package. Here's the design brief:- Dual microphone preamp inspired by the Ampex 351 circuit;
- Each section to combine elements of the Record and Repro boards of the original in a manner similar to the Electric Audio preamp mod;
- Tube complement of each section will be 2 x 12AX7 and 1 x 12AU7;
- Tubes provided will be New Old Stock (NOS) selected for low noise, gain, and balance;
- Custom wound transformers audio transformers from Sowter UK;
- Custom wound power transformer from Heyboer USA;
- 3U rack-mountable enclosure with laser-engraved faceplate;
- Black anodized aluminum contrasting with large high-quality brushed aluminum control knobs;
- Regulated 12.6VDC filament heat for all tubes;
- Tube rectifier replaced with silicon rectifier;
- Switchable between 120V/60Hz and 230Hz/50Hz power;
- Regulated 48VDC phantom power;
- Switchable -20 dB input attenuation on the XLR mic input;
- A separate 1/4' stereo jack line level input with fixed -30 dB attenuation;
- A front panel Mic/Line selector switch;
- A mono 1/4' instrument input directly to the input tube grids;
- Phase switch;
- Variable output attenuation using a Bourns 600Ω T-Pad Attenuator;
- Bypass switch for disengaging output attenuation;
- Primary gain control and gain trim adjustment;
- Ruggedized turret board construction for main amplifier boards;
- Custom hand-etched PCB for phantom and filament power supply;
- The unit will be equipped with an internal EMI/RF line power filter;
- Power supply features 105deg-rated low Z electrolytic capacitors for heat resistance and longevity;
- Signal capacitors are high-quality film types in the nF range, and silver mica in the pF range;
- Audio path wiring is aircraft-grade solid core 20AWG with PTFE insulation, shielded where necessary;
- Controls for gain and gain trim will utilize sealed 2W milspec potentiometers;
- 750V rated 2W metal film resistors will be used for low noise and durability;
- Where accurate balance is required for best CMRR, 0.1% precision resistors will be used;
- The ground scheme will be optimised for low noise, and will adhere to the AES48 standard to avoid the 'Pin 1 problem';
Audio Input Transformer
and the below description is only informative. In the last sentence also how to connect them.
Here's a pair of German made Beyerdynamic TR/BV 351 015 006 input transformers. These transformers have a single (not split) primary, a single secondary and a shield. The step up ratio is about 1:15 which translates to 200 ohms to 45,000 ohms, perfect for a tube microphone input, or moving coil step up to normal 47K phono stage impedance. The secondaries can also be terminated with high resistance up to about 100K without significant degradation of audio quality. Ampex used these transformers in several of their products (including the MX-10 tube mixer and nuvistor based MR-70 mic preamps) and simply attached the secondary directly to the grid with no other terminating resistor.
I find that with typical microphone levels and source impedances (100-200 ohms) the low end is flat to 20 Hz with low distortion into secondary loads as low as about 40K. The transformers' physical size precludes them being used with line levels, but as mic or MC phono transformers they are very tame and well refined. The top end is excellent. Most high ratio input transformers (even expensive ones like UTC, Triad, Peerless) tend to have a sharp rise above their high frequency spec (20,000 Hz or whatever) which results in ultrasonic ringing that you usually want to damp with a capacitor (or network) across the secondary. These actually have a gradual rolloff around 20,000 Hz, which results in a sweet treble sound even without special termination with a very good 1K square wave response, with very little overshoot or ringing.
Line Input Transformer
These transformers are removed from equipment, but are tested and working. They have approximately four inches of wire, so they're easy to get connected, and I'll also be sure to send you a pair with matching date codes.
Ampex 351 Input Transformer Schematic
The transformers have five wires, the red and black are the primary, the yellow and blue are secondary, the brown is the faraday shield. Red and blue are in phase with each other. If you need any additonal information, please don't hesitate to email. Thanks!
Mic Input Transformer
Questi trasformatori non sono in vendita. E solo una web personale dove pubblico un po di cose di audio. Sono così facile da collegare che no serve uno schema. 4 cavi, il beyer va collegato tra Denon DL103 e Preamp, sicuramente piu vicino possibile della Denon. Questo due volte, per due canali stereo.