A more realistic look at the Pultec style equalizer designs

One of the few historic audio devices with almost mystical status is the Pultec EQP-1A EQ and a lot of replicas has been made available during the decades. Whether being replicated in soft- or hardware, what can we expect from a more realistic point of view? Lets have a closer look.

Some fancy curves from the original EQP-1A manual
  • In the top most frequency range a shelving filter with 3 pre selected frequencies is offered but just for attenuation. Much more common and usable for todays mixing and mastering duties would be an air band shelving boost option here.
  • Also in the HF department there is just one single peak filter but this time just for boosting. It offers 7 pre selected frequencies between 3 and 16kHz and only here the bandwidth can be adjusted. However, the actual curves could have been steeper for todays mixing duties.
  • There is no option in the mid or low-mid range at all and also no high pass option. Instead, there is a shelving filter for the low-end which allows for boost and/or attenuation around four pre selected frequencies between 20 and 100 Hz.

All in all, this appears to be a rather quirky EQ concept with quite some limitations. On top of that, the low frequency behaviour of the boost and cut filters is rather unpredictable if both filters are engaged simultaneously which is exactly the reason why the original manual basically states “Do not attempt to do this!”.

Nowadays being refered to as the “Pultec Bass Trick” the idea is that you not only boost in some low end area but also create some sort of frequency dip sligthly above to avoid too much of a boost and muddiness in total. In practise, this appears to be rather unpredictable. Dial in a boost at 3 and an attenuation at 5, just as an example: Does this already feature a frequency dip? And if so at which frequency exactly? One has no idea and it even gets worse.

Due to aged electronics or component variety one has to expect that the actual curve behaviour might differ and also to see each vendors replica implementation to be different from another. In practise this indeed holds true and we can see the actual bass frequency dip at a much higher frequency within one model compared to another, just as an example.

… the more I boost the EQ the more it makes me smile …

A reviewers statement misguided by simple loudness increase?

Fun fact: Like the original device, all current (hardware) replica models do not have an output gain control. Also they increase the overall signal level just by getting inserted into the signal path.

So, where is the beef? Its definately not in the curves or the overall concept for sure. Maybe I’ll take some time for a follow-up article and a closer look into the buffer amplifier design to see if all the hype is justified.

Further Links

Not really demystifying but fun to read:

In the VoS plugin line you can find some Pultec style low end performance within NastyVCS: https://varietyofsound.wordpress.com/2010/05/07/nastyvcs-released-today/

Also interesting to read and hear: https://www.sweetwater.com/insync/pultec-shootout-with-sound-samples/

how old are your ears?

(via)

distortion in transformer cores, N. Partridge, 1939

This is the first of a series of articles in which the question of amplitude distortion arising in the iron core of a transformer will be dealt with on a quantitative basis. Information on this subject is scarce and the design data given is the outcome of original research by the author.

Be aware that tis articles appeared in the The Wireless World magazine from around 1939 (!). I’ve found some decent scans here: 1, 2, 3, 4.

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transformer circuit symbol

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