Traynor YBA-1 and Fender Hot Rod Deluxe, Tone Stack Comparison

In the last blog, I discussed the use of a Duncan Amp’s Tone Stack Calculator as a means to motivate the discussion of what makes a Marshall amplifier growl. Today, dear readers, I will back up that theoretical discussion by demonstrating measurements from both amplifiers, because really, what use is a tone stack calculator if it doesn’t *really* tell you about how an amp might sound?

For these experiments, the two amplifiers used are the Traynor Bassmaster YBA-1 and Fender Hot Rod Deluxe. If you really want me to measure a Marshall JTM45, then please, contact me, and send me one ;) These measurements were acquired using XX via the line out feature of a Hot Plate Attenuator. Signal generation and signal acquisition were performed by separate USB sound cards to hopefully eliminate the presence of ground loops or feedback affecting measurements. The amplifiers were driven in their linear regime before the onset of distortion. The nature of distortion will be discussed later and was excluded from this experiment for the sake of measuring only the tone circuit.

Fender Hot Rod Deluxe Frequency Response
Below, you will see plots of families of curves for the Fender Hot Rod Deluxe with one of the tone stack knobs (Bass, Mid, Treble) at levels of 0 (green curve), 50% (black curve), 100% (blue curve), while the others are kept at the mid level. The black curve has all knobs at level 5 and is repeated in all plots.

(a)

(b)

(c)

(d)

Figure 1. Frequency response of Fender Hot Rod compared with theoretical values. Tone control knobs are kept at mid-point if not being varied. (a) Varied bass response, (b) varied mid response, (c) varied treble response, and (d) the theoretical curve.

In making comparisons, one may at first be struck by the difference at the very low and very high end of the spectrum. Rest assured, this is due to the measurement system, which is a sound card running at 44.1 kHz. I have indicated this limitation in the sampling system with red lines in Figure 1d. The sound cards used cannot drive the low frequency signal very high, and sampling at 44.1 kHz only allows samples of signals up to about 22 kHz. This is known as the Nyquist theorem to the engineer but could just as easily be known as the rule of “twice as much is needed to make a useful measurement.” In other words, you need at least twice the sampling rate to capture a signal at half the sampling rate. I will discuss this in more detail in another blog, because some people might be wondering: why are sound cards using 96 kHz sold if humans only hear up to 20 kHz (which requires the 40 kHz sampling rate). I can assure you, the answer is not so we can record dog whistles, but this will come in due time.

Traynor Bassmaster YBA-1
Below, you will see plots of families of curves for the YBA-1 with one of the tone stack knobs (Bass, Mid, Treble) at levels of 0 (green curve), 5 (black curve), 10 (blue curve). The black curve has all knobs at level 5 and is repeated in all plots.

(a)

(b)

(c)

(d)

Figure 2. Frequency response of Traynor YBA-1 compared with theoretical values. Tone control knobs are kept at mid-point if not varied. (a) Varied bass response, (b) varied mid response, (c) varied treble response, and (d) the theoretical curve.

Okay. What the hell is going on with the bass response on the Traynor measurements? The bass frequencies in the theoretical model are almost equal in amplitude to the high frequencies. This is clearly not the case for “guitar” channel on the Traynor. Since the YBA-1 was made for bass, my suspicion is that the Traynor engineers placed a low-cut filter at the "guitar" input of the amplifier in order to make the channel more ‘bright’ and ‘guitar-like’. Next time, readers, this will all be clarified with schematics and measurements. Possibly this is the only place where the Traynor is lacking in the ‘growl’ department—crappy bass. I mean, the HIGHs are greatly outweighing the LOWs in the Traynor as I am currently playing it through the guitar input as measured above.