Analysing The GT2 & DRP-1 – active filter sections

1 Introduction.......................................................................................................................................2 2 Circuit analysis..................................................................................................................................3

2.1 GT-2 structure ............................................................................................................................3 2.2 DRP-1 structure..........................................................................................................................4 2.3 Comment....................................................................................................................................4

3 Simulations........................................................................................................................................5

Analysing The GT2 & DRP-1 – active filter sections

1 Introduction This document will present the results of a PSpice frequency analysis of the two filter sections used for reducing the harmonic content of the clipped signal in the Sans Amp GT2 manufactured by Tech 21 and the DRP-1 manufactured by Marshall. Both products are guitar amp simulators used for direct recording or live performances, and therefore both products simulate preamp, tonestack, poweramp and speakersection. Note that the GT2 also simulates various preamps and the change in frequency response when moving a mic from edge of speaker to cone. The simulations are based on the following schematics: Marshall DRP-1 found at www.GodiksenNet.com Sans Amp GT-2 found at www.TonePad.com

Analysing The GT2 & DRP-1 – active filter sections

2 Circuit analysis

2.1 GT-2 structure

Figur 1 GT2 structure The GT-2 can emulate various classic guitar amplifiers with different frequency response and gain structures. This is accomplished using the above structure, which again can be divided into the following sections: Pre filtering: this section consists of BP1 and Gain Stage 1. The main purpose of this section is to produce frequency responses related to the 3 amp type: California, British and Tweed. Please note that s3 is used to switch between the 3 amp types. Gain and clipping stage: The Drive control is included in this section, which controls the amplitude of the signal feeding BP 2. The main purpose of this section is to produce an output signal with a higher harmonic content; this is accomplished with BP 2 driving Gain Stage 2 into clipping. Please note that s2 is used to change gain mode and voicing from Clean, Hi-gain to Hot Wired. Filtering stage: LP 1 is low pass filter used to dampen unwanted higher order harmonics from the clipped signal from Gain Stage 2. This section is the target for this analysis. Mic placement simulator: This section is a passive filter emulating the frequency response of a microphone in 3 different positions in front of the speaker. Please note that s1 is used to change filter settings from Off axis, Center to Classic. Master tone control: This block is used to change the overall tone of the unit. The GT-2 is using the same gain and clipping stage, followed by a filtering stage emulating various guitar amplifiers. This approach, from my point of view, means that this unit primarily emulates the distortion produced in only one section of a guitar amplifier. Comparing with the DRP-1 which is emulating preamp and poweramp sections – se section 2.2 - reveals this fact. The main reason for the convincing sounds produced by this unit is a very well designed pre filtering section, which in combination with the gain stage and filtering stage produces a signal with the same harmonic content as a preamp, tonestack and poweramp of a classic amp. Having said this, the engineers who designed this unit, must have had the real classic amps analysed using spectrum analysers in order to get the needed information. The same goes for the design of the mic placement emulator.

Analysing The GT2 & DRP-1 – active filter sections

2.2 DRP-1 structure

Figur 2 DRP-1 structure The DRP-1 is – when compared to the GT-2 – a one trick pony, emulating a complete Marshall type of amp. This means that the DRP-1 emulates the gain structure and frequency response of preamp, poweramp and a cabinet with speaker. Preamp: Gain Stage 1, BP 1 and Gain Stage 2 in conjunction with LED Clip 1 emulates the performance of a tube preamp, including frequency response and harmonic content when overdriven. These sections are followed by a passive tone stack with a volume control completing the preamp section. The preamp section has a built in boost circuit, which increases gain in the section driving the tone stack. The boost function also inserts 2 LED’s used for clipping the signal right before the tone stack. Power amp: This section emulates the performance of a tube power amp, including frequency response and harmonic content when overdriven. The power amp section is made by using a single gain stage driving a clipping section made by 2 LED’s. The volume of the output signal is controlled by a volume control. The gain stage has a variable tone control, controlling the frequency response of the signal being clipped. Filtering stage: This section is a low pass filter used to dampen unwanted higher order harmonics from the clipped signal from the power amp. This means that the output signal should sound like a miked Marshall tube amp. This section is the target for this analysis. Note that the DRP-1 is having more than one clipping stage, meaning it will have an amp like performance; it is possible to use it with low preamp gain and instead have the feeling of an overdriven power amp. Of Course the opposite situation is an option too.

2.3 Comment Looking at the GT-2 described in section 2.1 figur 1, and removing the Mic emulator and the Tone Control, comparing with the DRP-1 described in section 2.2 it is obvious that the LP filter in both circuits fulfil the same demands – damping unwanted higher order harmonics from the clipped signal before delivering the signal. In the DRP-1 one could suspect that the purpose of the circuit to emulate the frequency response of a generic speaker, and in the GT-2 likewise. Having this in mind, I decided to examine these filters in both the DRP-1 and the GT-2. The result of my simulations is shown in section 3.

Analysing The GT2 & DRP-1 – active filter sections

3 Simulations

VEE0

15.00VC108

220n

-10.65uV

-21.63uV

VCC

U1A

TL072/301/TI

3

2

84

1+

-

V+

V-

OUT

DRP-1 The speaker emulator

-15.00V

R105

10k

0V

-674.3nV

0V

R106

10k-21.18uV

-15.00V

-11.65uV

R104

470k

R103

12k

C111

2.2n Rload_DRP1

100k

Vout_DRP1

15.00V

-11.20uV

VCC

0

U2A

TL072/301/TI

3

2

84

1+

-

V+

V-

OUT 0VV31Vac0Vdc

C_out_DRP1

10u

0

R107

12k

0

0V

C109

10n

C112

4.7n

0V

VEE

-21.18uV

0V0

C110

1n

Figur 3 DRP-1 Speaker emulator

0

-2.974uV

-15.00V

5.024uV

VEE

C12

1n

7.997uV

C8

560p

C_out_GT2

10u

0

U4A

TL072/301/TI

3

2

84

1+

-

V+

V-

OUT

R4

33k

V41Vac0Vdc

Cg

220n

-1.982uV

Vout_GT2

0V

0

R16

10k

R7

22k

0VVEE

VCC

0V

C9

10n

0V

15.99uV

VCC

Rload_GT2

100k

0

15.00VR6

22k

R15

33k

GT-2The speaker emulator

-15.00V

U3A

TL072/301/TI

3

2

84

1+

-

V+

V-

OUT

0V

R8

22k

C7

47n

6.511uV-1.982uV

C13

2.2n

0

0V

-991.2nV

0

-991.2nV

15.00V

R5

10k

0V

Figur 4 GT2 Speaker emulator The above circuits are placed in the signal path after the clipping stages. The purpose of both emulators is to remove unwanted higher order harmonic content from the clipped signal, resulting in a signal with a harmonic content similar to the signal from a mic’ed tube guitar amplifier. Basically, the DRP-1 and the GT2 circuits both are 4 order Sallen Key lowpass filters – implemented by using 2 2 order Sallen Key lowpass filters. Only difference is the GT2 having an extra pole and zero made by R5 and C7. The GT2 also has a passive 6 dB attenuation stage at the input in order to prevent the big voltage swing from the clipping op-amp overdriving the filter. The 6 dB attenuation is made by R8 in parallel with the filter Zin, and R16.

Analysing The GT2 & DRP-1 – active filter sections

Frequency

1.0Hz 3.0Hz 10Hz 30Hz 100Hz 300Hz 1.0KHz 3.0KHz 10KHz 30KHz 100KHzVdb(Vout_DRP1) Vdb(Vout_GT2)

-60

-40

-20

-0

-70

Figur 5 Frequency response, DRP-1 and GT-2 The DRP-1 has a standard lowpass Chebychev frequency characteristic, with a peak. The GT2 has max peak at – 6 dB, before having a fairly straight – 15 dB damping, before rolling off without a peak.