Guitar Preamp and 'Flanger' Using PT2399 Time Delay IC.


I have just finished making a guitar amplifier starting with an 'ADS mixer amplifier' I bought on eBay. There is a row of 7 controls on the front panel, and I wanted to use 2 for a 'flanger' type effect. Many years ago I experimented with a reel to reel tape recorder using two replay heads connected in series and with a mechanical lever to vary the length of tape between the heads. The result was impressive, but looking at current designs it is invariably stated that the time delay between the two added signals should be just a few msec for this effect. There is an easily available and low cost time delay IC, the PT2399, which looks interesting, but has a minimum delay around 30ms, which may be a problem. My tape effect certainly had a far higher delay and still produced a good result, so some experimentation seemed worthwhile. There are a few examples of flangers using this device, but they use a pair for the two signal paths so that the delay difference can be small or pass through zero.

A good source of information on using the PT2399 time delay IC is Project 26A on the Elliot Sound Products website, and the components attached to the PT2399 time delay IC are more or less a direct copy from Fig.3 on that page, but using whatever component values I had available. The only significant addition is the BC549C transistor which is used to modulate the time delay. The base current of the transistor is supplied by a variable frequency oscillator IC2a, which is a TLC082, which has a CMOS input and BJT output. This has a 22n connected between the supply pins, - close to the IC as suggested in the data sheet to maintain stability.

IC1a is the input stage and adjustable clipper. The 500R preset adjusts the sensitivity, I used it set to maximum resistance. IC2b sums the direct signal and the time delayed version. Setting Vr3 to maximum gives maximum flanging effect, and setting it to minimum leaves just the direct signal. The output stage is a conventional tone control with bass, mid and treble controls. This is copied from page 13 of the LF353 data, once more using whatever component values I had available.

Vr2 and Vr3 should probably be linear, but I only had log available. The 7 controls were the original controls from an ADS30 mixer amplifier I bought on eBay for 6. I removed everything apart from the transformer and bridge rectifier and added a fairly basic lateral mosfet amplifier. There is a row of seven 50k controls on the front panel, and these are all used in the above circuit. Only the tone controls were linear. I also kept the original heatsink, but moved it from inside the case to outside on the back panel.

Having built the circuit I expected to need some experimentation to optimise some component values, but the first impressions were better than expected, and nothing is in need of any obvious improvement. Checking with an oscilloscope the modulation oscillator output at the collector of the BC549C was an approximate triangle wave, but the negative peak is flattened. If required that could be avoided by increasing the 2M7 base resistor, e.g. to 3M3 or 3M9.

So what does it sound like? The time delay is longer than usually chosen for flangers, and the result is perhaps more warble than flange, but even so there are some interesting effects. I will try to add some sample sound files when I can get my tame guitarist to perform something. My own guitar playing talent is close to zero.

With the addition of a switch and a few components an 'echo' effect can be produced, but still with a modulated delay, so the effect is a little unusual. The modulation oscillator could be disabled for the echo effect if a double pole switch was used.

The power amplifier I added to the ADS case is nothing special, just a simple basic mosfet amplifier, it is probably not unconditionally stable into any load, it is only intended for use with a standard guitar speaker via a short connecting cable, and significantly capacitive loads should be avoided. Originally I had a 100u across the 12V zener in the input stage, but the switch-on thump was then rather extreme, so that capacitor had to go. Now there is still a noticeable clunk at switch on or off, but not too bad. Some experiment with the value of the other 100u could probably reduce the problem further. The circuit is shown next. Note there is no quiescent current adjustment, I just chose the 200R resistor in the bias chain to give a 100mA output stage current, so with different mosfets these could need some adjustment to the 200R. I previously have only used Renesas mosfets, these Exicon types appear to need slightly higher Vgs at 100mA than the previous types, but all are well inside the specified range. The maximum output power should be around 40W.


References

TLC082 CMOS Input Dual Opamp.
OPA2134 Dual Opamp.
PT2399 Time Delay.
BC549C npn transistor.

Photos


HOME