Fully Parametric Tube EQ (with gyrators)

Description: A fully parametric tube equalizer (EQ) design aims to merge two schematics into one. The goal is to create a tube-based gyrator section with adjustable Q, frequency, and gain while utilizing integrated circuits (ICs) for the input and follower stages. This approach is intended for use in a home-built mixer, minimizing the number of tubes. The schematic serves as a learning tool. The designer replaced a transistor with a tube and attempted simulation using a Spice program, but faced challenges due to inexperience. There is curiosity about the Electro Harmonix Tube EQ pedal, which reportedly employs a "detuned Twin-T" format, although the specific tube or IC components are unclear. A question arises regarding a 0.5 MΩ potentiometer connected to the grid of V2, which is unmarked; it is suspected to be a boost/cut control for the bass section. Additionally, the placement of a boost-cut potentiometer for the treble stage is questioned, as it is not visible in the schematic. The design is intended for two channels, with the treble channel potentially being omitted in the initial version. The cathodyne phase inverter is discussed, particularly its output signals and feedback loop characteristics. The designer suggests using a blend pot from both outputs of the cathodyne to the input stage of a 1955 circuit, with feedback from the tone control. Caution is advised regarding positive feedback to avoid oscillation. There is speculation about the performance of varying between inverted and non-inverted signals in a Twin-T filter and the challenges of experimenting due to relocation. The cathode follower's operation under negative feedback is noted, and the possibility of enhancing gain through feedback is mentioned. The designer expresses a desire to progress with the project upon returning to their electronics equipment.

A fully parametric tube equalizer (EQ) design is a sophisticated audio processing circuit that allows for precise control over the frequency response of an audio signal. This design aims to merge two existing schematics into a single, cohesive unit, integrating a tube-based gyrator section that provides adjustable parameters such as Q (quality factor), frequency, and gain. The use of integrated circuits (ICs) for the input and follower stages is intended to reduce the number of vacuum tubes required, which is particularly beneficial for a home-built mixer application.

The schematic serves as a foundation for learning and experimentation in circuit design. In this context, a transistor has been replaced with a vacuum tube, a common practice in analog audio design to impart a characteristic warmth to the sound. However, initial simulation efforts using a Spice program have not yielded successful results, likely due to the designer's limited experience with circuit simulation tools.

The Electro Harmonix Tube EQ pedal is referenced as a point of interest, noted for its use of a "detuned Twin-T" filter configuration. This type of filter is known for its notch filtering capabilities and is often employed in EQ designs to selectively attenuate specific frequencies while allowing others to pass unaltered. The exact configuration of tubes and ICs within this product remains unspecified.

A notable component in the schematic is a 0.5 MΩ potentiometer connected to the grid of vacuum tube V2. This component is presumed to act as a boost/cut control for the bass frequency section of the EQ. The absence of a similar potentiometer for the treble section raises questions about its implementation and design.

The two-channel design allows for flexibility, with the option to defer the development of the treble channel in the initial iteration. The cathodyne phase inverter is a critical element in this design, providing both inverted and non-inverted output signals. The feedback loop characteristics of a cathodyne are discussed, highlighting the potential for feedback to enhance gain while also necessitating careful management to avoid oscillation.

To integrate the outputs of the cathodyne into a subsequent stage, the designer proposes using a blend pot connected to both outputs, feeding the signal into the input stage of a 1955 circuit. This method allows for fine-tuning the balance between dry and processed signals. It is emphasized that the design should maintain low insertion loss, reducing the need for additional recovery stages.

The exploration of varying between inverted and non-inverted signals in a Twin-T filter is suggested as an experimental avenue, although practical testing may be hindered due to the designer's current circumstances. The operation of a cathode follower under negative feedback conditions is acknowledged, with the potential for enhanced gain through strategic feedback configurations.

In conclusion, the design of a fully parametric tube EQ involves a complex interplay of analog components, with careful consideration given to the interaction of various circuit elements. The schematic serves as both a practical guide and a learning tool for those venturing into the realm of tube-based audio processing.A fully parametric tube EQ and my best bet seems to be `merging` these two schematics into one so that i can get a tube-based gyrator section (with adjustable Q, Freq and Gain) and use ICs for the input and follower stages (going to use this on a home-built mixer so reducing number of tubes is the reason). I did this schematic in hopes of understanding things better. I basically just replaced the transistor with the tube. I tried simulating it in a Spice program but didn`t work for me - i`ve never tried simulation before so probably a PBCAT issue (problem between chair and terminal). I am totally new to `designing` any circuits, so this schematic may have some laughable elements, but hey, gotta start somewhere.

By the way - i hear that the Electro Harmonix Tube EQ pedal uses a "detuned Twin-T" type format, tho i don`t what parts of theirs are tube or IC (nor what they mean by `detuned`). my one question is about boost-cut. on the schem there`s a lone. 5meg pot connected to the grid of V2 that is unmarked (to the upper-left of V2). is this boost/cut for the bass section and secondly, where does a boost-cut pot go for the treble stage (i don`t see one) Since that`s a 2-channel, the treble channel need not be made in the first go-round, and the cathodyne could be made from the first triode and the EQ powered from the second, in the same envelope.

That gives 1 mini tube socket per channel. When you mean the feedback loop of a cathodyne-do you mean a cathode follower-but I didnt think a cathode follower had a feedback loop am I being dumb-am I missing something I don`t have the ability to frame it into the original circuit, but a cathodyne is a phase inverter that pulls a non-inverted signal from the cathode, like a cathode follower, and an inverted signal from the anode, like a standard gain stage, from the same triode. Read Merlin Blencowe`s site (and book for a MUCH bigger explanation!) at Valve Wizard - How to design valve guitar amplifiers.

Rk and Rp (Ra using British terminology) are normally the same value of ~100k or so, take a little bit maybe for a cathode bias resistor, so the balance between the two signals is usually really close. What I meant was to hook up a blend pot from both outputs of the cathodyne to the input stage of the 1955 circuit, then take a signal from the output of the tone control and feed it back into the input of the cathodyne.

At dead center of the pot you should get NO signal from the circuit as a whole, so it would be best to run it in parallel with a dry signal. You would have to be very careful with the positive feedback because of the possibility of oscilllation.

You would also take the signal to the next stage from the usual output. You would probably not need a recovery stage because there shouldn`t really be any insertion loss. I`ve never actually tried to vary between an inverted and non-inverted signal into a twin-T filter (which is a notch filter at heart) so I`m not sure how well it would really work. I`m moving out of state right now and most of my electronics stuff is staying where it is so trying it will be impossible for me.

It`s just a suggested experiment so take it with a grain of salt. A cathode follower operates under 100% negative feedback, and any time you hook up an output back to its own input, you`ve created a feedback loop. Even more fun with a cathode follower is hooking its output up to bootstrap the gain stage directly in front of it -> gain of the gods, also covered in the Blencowe book on preamps.

Worth a read and re-read for a beginner! hey ValvusMusicus, did you move forward with this at all I am back in the country with all my project stuff again and am hoping to tackle this thing finally! GCA - thanks for explaining that. I have to admit, following a schematic is one thing, but I am pretty daunted by not only building this but also implementing something heretofore i had never heard of